Allow YAML config file to contain None (#7779)

Useful when config file is fully commented

Signed-off-by: Alex Kotov <kotovalexarian@gmail.com>

changelog.d/7779.bugfix

@@ -0,0 +1 @@
+Fix synctl to handle empty config files correctly. Contributed by @kotovalexarian.

synctl

@@ -239,7 +239,8 @@ def main():
     for config_file in config_files:
         with open(config_file) as file_stream:
             yaml_config = yaml.safe_load(file_stream)
-        config.update(yaml_config)
+        if yaml_config is not None:
+            config.update(yaml_config)
 
     pidfile = config["pid_file"]
     cache_factor = config.get("synctl_cache_factor")