frontend: return empty dict instead of null in API calls to BE

get_build_record and get_srpm_build_record methods returns
None in case of exception or empty task which results in
null response in API endpoints. Backend worker
don't expect null value, tries to unpack it and fails

frontend/coprs_frontend/coprs/views/backend_ns/backend_general.py

@@ -354,6 +354,9 @@ def get_build_task(task_id):
     try:
         task = BuildsLogic.get_build_task(task_id)
         build_record = get_build_record(task)
+        if build_record is None:
+            build_record = {}
+
         return flask.jsonify(build_record)
     except CoprHttpException as ex:
         jsonout = flask.jsonify({"msg": str(ex)})
@@ -370,6 +373,9 @@ def get_srpm_build_task(build_id):
         jsonout.status_code = 404
         return jsonout
     build_record = get_srpm_build_record(task)
+    if build_record is None:
+        build_record = {}
+
     return flask.jsonify(build_record)