Fix importlib.metadata.files workaround

Some distributions, such as Fedora, do not install the RECORDS file as
part of a package's dist-info. As a result importlib.metadata.files will
return None.

Use the workaround for these cases as well.

Signed-off-by: Felix Kaechele <felix@kaechele.ca>

borgmatic/config/validate.py

@@ -16,16 +16,16 @@ def schema_filename():
     '''
     Path to the installed YAML configuration schema file, used to validate and parse the
     configuration.
-
-    Raise FileNotFoundError when the schema path does not exist.
     '''
+
+    files = importlib_metadata.files('borgmatic')
+    if files is not None:
         try:
-        return next(
-            str(path.locate())
-            for path in importlib_metadata.files('borgmatic')
-            if path.match('config/schema.yaml')
-        )
+            return next(str(path.locate()) for path in files if path.match('config/schema.yaml'))
         except StopIteration:
+            # schema not found in package, fall through to the approach below
+            pass
+
     # If the schema wasn't found in the package's files, this is probably a pip editable
     # install, so try a different approach to get the schema.
     return os.path.join(os.path.dirname(borgmatic.config.__file__), 'schema.yaml')