fix: Test Compatibility returns true when schema missing

To make the api compatible with Confluent Schema Registry, testing the schema compatibility should return "true" wheneber the subject, schema or the version is missing

src/karapace/schema_registry_apis.py

@@ -1354,14 +1354,8 @@ def get_old_schema(self, subject: Subject, version: Version, content_type: str)
         except InvalidVersion:
             self._invalid_version(content_type, version)
         except (VersionNotFoundException, SchemasNotFoundException, SubjectNotFoundException):
-            self.r(
-                body={
-                    "error_code": SchemaErrorCodes.VERSION_NOT_FOUND.value,
-                    "message": f"Version {version} not found.",
-                },
-                content_type=content_type,
-                status=HTTPStatus.NOT_FOUND,
-            )
+            self.r({"is_compatible": True}, content_type)
+
         old_schema_type = self._validate_schema_type(content_type=content_type, data=old)
         try:
             old_references = old.get("references", None)