fixes null server errors for tests

Author: ValJed

modules/@apostrophecms/modal/ui/apos/mixins/AposEditorMixin.js

@@ -31,7 +31,7 @@ export default {
       docFields: {
         data: {}
       },
-      serverErrors: null,
+      serverErrors: {},
       restoreOnly: false,
       readOnly: false,
       changed: [],

modules/@apostrophecms/schema/ui/apos/components/AposSchema.vue

@@ -69,7 +69,7 @@
         :modifiers="fields[field.name].modifiers"
         :display-options="getDisplayOptions(field.name)"
         :trigger-validation="triggerValidation"
-        :server-error="fields[field.name].serverError"
+        :server-error="serverErrors[field.name]"
         :doc-id="docId"
         :generation="generation"
         @update-doc-data="onUpdateDocData"

modules/@apostrophecms/schema/ui/apos/logic/AposSchema.js

@@ -81,7 +81,7 @@ export default {
     serverErrors: {
       type: Object,
       default() {
-        return null;
+        return {};
       }
     },
     displayOptions: {