Do not fail on empty YAML file

Change-Id: Ia12862f902c2f0b638fcbff798553ca404d81e24

jenkins_jobs/loader.py

@@ -119,6 +119,8 @@ def load_files(config, roots, path_list):
             data = loader.load_fp(path)
         else:
             data = loader.load_path(path)
+        if data is None:
+            continue
         if not isinstance(data, list):
             raise JenkinsJobsException(
                 f"The topmost collection must be a list, but is: {data}",

tests/yamlparser/job_fixtures/empty_template_file.yaml

@@ -0,0 +1,2 @@
+# Nothing is here.
+# Just check that jenkins-jobs do not fail on YAML file with no contents.