Don't crash when one of the configuration files is empty (matrix-org#7341)

If the admin adds a `.yaml` file that's either empty or doesn't parse into a dict to a config directory (e.g. `conf.d` for debs installs), stuff like matrix-org#7322 would happen. This PR checks that the file is correctly parsed into a dict, or ignores it with a warning if it parses into any other type (including `None` for empty files).

Fixes matrix-org#7322

Author: babolivier

changelog.d/7341.bugfix

@@ -0,0 +1 @@
+Fix bad error handling that would cause Synapse to crash if it's provided with a YAML configuration file that's either empty or doesn't parse into a key-value map.

synapse/config/_base.py

@@ -657,6 +657,12 @@ def read_config_files(config_files):
     for config_file in config_files:
         with open(config_file) as file_stream:
             yaml_config = yaml.safe_load(file_stream)
+
+        if not isinstance(yaml_config, dict):
+            err = "File %r is empty or doesn't parse into a key-value map. IGNORING."
+            print(err % (config_file,))
+            continue
+
         specified_config.update(yaml_config)
 
     if "server_name" not in specified_config: