[Ingest pipelines] Accept 0, false and '' in the set processor value field (elastic#246032)

Fixes elastic#220572

## Summary
This PR fixes a bug in the Set processor form in the Ingest Pipelines
plugin. Previously, the value field incorrectly rejected falsy values
like 0, false, and empty strings (''), treating them as invalid/empty.

### How to test
* Navigate to edit pipelines UI and create a Set processor.
* Verify that you can save the processor adding the following to the
`Value` field: `0`, `false` or empty.
* Verify that the `copy_from` still not accept empty values.
* Import a processor like the following value and verify that the info
is correctly displayed when editing the processor (try also to change
the value field to `false` and `''`):

```
{
  "description": "test",
  "processors": [
    {
      "set": {
        "field": "test",
        "value": 0
      }
    }
  ]
}

```

Co-authored-by: Elena Stoeva <[email protected]>

Author: SoniaSanzV

x-pack/platform/plugins/shared/ingest_pipelines/public/application/components/pipeline_editor/__jest__/processors/set.test.tsx

@@ -55,10 +55,7 @@ describe('Processor: Set', () => {
     await saveNewProcessor();
 
     // Expect form error as "field" is required parameter
-    expect(form.getErrorsMessages()).toEqual([
-      'A field value is required.',
-      'A value is required.',
-    ]);
+    expect(form.getErrorsMessages()).toEqual(['A field value is required.']);
   });
 
   test('saves with default parameter value', async () => {
@@ -193,4 +190,59 @@ describe('Processor: Set', () => {
       value: { value_1: 'aaa\"bbb', value_2: 'aaa(bbb' },
     });
   });
+
+  test('saves with empty string as value', async () => {
+    const {
+      actions: { saveNewProcessor },
+      form,
+    } = testBed;
+
+    form.setInputValue('fieldNameField.input', 'field_1');
+
+    form.setInputValue('textValueField.input', '');
+
+    await saveNewProcessor();
+
+    const processors = getProcessorValue(onUpdate, SET_TYPE);
+    expect(processors[0][SET_TYPE]).toEqual({
+      field: 'field_1',
+      value: '',
+    });
+  });
+
+  test('saves with "0" as value', async () => {
+    const {
+      actions: { saveNewProcessor },
+      form,
+    } = testBed;
+
+    form.setInputValue('fieldNameField.input', 'field_1');
+
+    form.setInputValue('textValueField.input', '0');
+
+    await saveNewProcessor();
+
+    const processors = getProcessorValue(onUpdate, SET_TYPE);
+    // "0" is JSON-parsed to the number 0 during serialization
+    expect(processors[0][SET_TYPE].field).toEqual('field_1');
+    expect(processors[0][SET_TYPE].value).toEqual(0);
+  });
+
+  test('saves with "false" as value', async () => {
+    const {
+      actions: { saveNewProcessor },
+      form,
+    } = testBed;
+
+    form.setInputValue('fieldNameField.input', 'field_1');
+
+    form.setInputValue('textValueField.input', 'false');
+
+    await saveNewProcessor();
+
+    const processors = getProcessorValue(onUpdate, SET_TYPE);
+    // "false" is JSON-parsed to the boolean false during serialization
+    expect(processors[0][SET_TYPE].field).toEqual('field_1');
+    expect(processors[0][SET_TYPE].value).toEqual(false);
+  });
 });

x-pack/platform/plugins/shared/ingest_pipelines/public/application/components/pipeline_editor/components/processor_form/processors/set.tsx

@@ -94,9 +94,6 @@ const fieldsConfig: FieldsConfig = {
     deserializer: (value: string | object) => {
       return isXJsonValue(value) ? to.xJsonString(value) : value;
     },
-    serializer: (value: string) => {
-      return isXJsonValue(value) ? value : from.emptyStringToUndefined(value);
-    },
     label: i18n.translate('xpack.ingestPipelines.pipelineEditor.setForm.valueFieldLabel', {
       defaultMessage: 'Value',
     }),
@@ -109,7 +106,9 @@ const fieldsConfig: FieldsConfig = {
     validations: [
       {
         validator: ({ value, path, formData }) => {
-          if (isEmpty(value) && isUndefined(formData['fields.copy_from'])) {
+          // Only require a value if it's undefined and copy_from is also not defined.
+          // Empty strings, 0, and false are valid values.
+          if (isUndefined(value) && isUndefined(formData['fields.copy_from'])) {
             return {
               path,
               message: i18n.translate('xpack.ingestPipelines.pipelineEditor.requiredValue', {
@@ -220,19 +219,20 @@ export const SetProcessor: FunctionComponent = () => {
         })}
       />
 
-      <UseField
-        config={fieldsConfig.value}
-        component={XJsonToggle}
-        path="fields.value"
-        componentProps={{
-          disabled: isCopyFromEnabled,
-          handleIsJson: getIsJsonValue,
-          fieldType: 'text',
-        }}
-        validationData={isDefineAsJson}
-      />
+      {!isCopyFromEnabled && (
+        <UseField
+          config={fieldsConfig.value}
+          component={XJsonToggle}
+          path="fields.value"
+          componentProps={{
+            handleIsJson: getIsJsonValue,
+            fieldType: 'text',
+          }}
+          validationData={isDefineAsJson}
+        />
+      )}
 
-      {hasTemplateSnippet(fields?.value) && (
+      {!isCopyFromEnabled && hasTemplateSnippet(fields?.value) && (
         <UseField
           componentProps={{
             euiFieldProps: {