Fix: allow missing fields field in SaladRecordSchema (#551)

* Fix: allow missing `fields` field in SaladRecordSchema

The field SaladRecordSchema::fields is optional. see:
 - https://www.commonwl.org/v1.2/SchemaSalad.html#SaladRecordSchema
 - https://github.com/common-workflow-language/schema_salad/blob/d123d9c81fd22337e37a0b7e59f92019b0fdae3f/schema_salad/metaschema/metaschema_base.yml#L111

Author: SGSSGene

schema_salad/avro/schema.py

@@ -520,10 +520,7 @@ def __init__(
         other_props: Optional[PropsType] = None,
     ) -> None:
         # Ensure valid ctor args
-        if fields is None:
-            fail_msg = "Record schema requires a non-empty fields property."
-            raise SchemaParseException(fail_msg)
-        elif not isinstance(fields, list):
+        if not isinstance(fields, list):
             fail_msg = "Fields property must be a list of Avro schemas."
             raise SchemaParseException(fail_msg)
 
@@ -622,7 +619,7 @@ def make_avsc_object(json_data: JsonDataType, names: Optional[Names] = None) ->
                     symbols = cast(List[str], symbols)
                 return EnumSchema(name, namespace, symbols, names, doc, other_props)
             if atype in ["record", "error"]:
-                fields = json_data.get("fields")
+                fields = json_data.get("fields", [])
                 if not isinstance(fields, list):
                     raise SchemaParseException(
                         '"fields" for type {} must be a list of mappings: {}'.format(

schema_salad/tests/test_java_codegen.py

@@ -5,7 +5,7 @@
 from schema_salad import codegen
 from schema_salad.schema import load_schema
 
-from .util import cwl_file_uri, metaschema_file_uri, get_data
+from .util import cwl_file_uri, get_data, metaschema_file_uri
 
 
 def test_cwl_gen(tmp_path: Path) -> None:

schema_salad/tests/test_misc.py

@@ -0,0 +1,11 @@
+from schema_salad.avro.schema import Names
+from schema_salad.schema import load_schema
+
+from .util import get_data
+
+
+def test_misc() -> None:
+    path = get_data("tests/test_schema/no_field_schema.yml")
+    assert path
+    document_loader, avsc_names, schema_metadata, metaschema_loader = load_schema(path)
+    assert isinstance(avsc_names, Names)

schema_salad/tests/test_python_codegen.py

@@ -8,7 +8,7 @@
 from schema_salad.avro.schema import Names
 from schema_salad.schema import load_schema
 
-from .util import cwl_file_uri, metaschema_file_uri, basket_file_uri
+from .util import basket_file_uri, cwl_file_uri, metaschema_file_uri
 
 
 def test_cwl_gen(tmp_path: Path) -> None:

schema_salad/tests/test_schema/no_field_schema.yml

@@ -0,0 +1,3 @@
+- name: EmptyType
+  documentRoot: true
+  type: record

schema_salad/tests/util.py

@@ -2,6 +2,7 @@
 from typing import Optional, Text
 
 from pkg_resources import Requirement, ResolutionError, resource_filename
+
 from schema_salad import ref_resolver