mr: check for pipeline key in api missing, or existing but being None

Fixes #25

Inspired from hexaheximal

mrhlpr/mr.py

@@ -136,7 +136,7 @@ def get_pipeline_metadata(
     # Get the latest pipeline (without cache so we don't miss newer ones)
     url_mr = "/projects/{}/merge_requests/{}".format(origin.api_project_id, mr_id)
     api = gitlab.download_json(origin, url_mr, no_cache=True)
-    if "pipeline" not in api:
+    if api.get("pipeline") is None:
         logging.error("ERROR: no pipeline found in merge request")
         exit(1)