fix: when instantiation type is not defined for language, return type itself for parent (OpenAPITools#18164)

Author: mvarchdev

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -2059,7 +2059,11 @@ public String toInstantiationType(Schema schema) {
         if (ModelUtils.isMapSchema(schema)) {
             Schema additionalProperties = ModelUtils.getAdditionalProperties(schema);
             String inner = getSchemaType(additionalProperties);
-            return instantiationTypes.get("map") + "<String, " + inner + ">";
+            String mapInstantion = instantiationTypes.get("map");
+            if (mapInstantion != null) {
+                return mapInstantion + "<String, " + inner + ">";
+            }
+            return inner;
         } else if (ModelUtils.isArraySchema(schema)) {
             ArraySchema arraySchema = (ArraySchema) schema;
             String inner = getSchemaType(getSchemaItems(arraySchema));