Avoid error in FileInfo repr due to permissions being None (#956)

FileInfo.__repr__ was raising a TypeError due to the expectation that
permissions was never None, and the ":o" format string:

TypeError: unsupported format string passed to NoneType.__format__

At first I was surprised that Pyright didn't catch this, because
clearly dict.get can return None, but we were defining a _FileKwargs
TypedDict with a non-optional "permissions" field, so fix that too.

Fixes #955

ops/testing.py

@@ -66,7 +66,7 @@
     _StringOrPath = Union[str, pathlib.PurePosixPath, pathlib.Path]
     _FileOrDir = Union['_File', '_Directory']
     _FileKwargs = TypedDict('_FileKwargs', {
-        'permissions': int,
+        'permissions': Optional[int],
         'last_modified': datetime.datetime,
         'user_id': Optional[int],
         'user': Optional[str],
@@ -2476,7 +2476,7 @@ def get_pebble_file_type(file: '_FileOrDir') -> pebble.FileType:
                 name=file.name,
                 type=get_pebble_file_type(file),
                 size=file.size if isinstance(file, _File) else None,
-                permissions=file.kwargs.get('permissions'),
+                permissions=file.kwargs.get('permissions') or 0,
                 last_modified=file.last_modified,
                 user_id=file.kwargs.get('user_id'),
                 user=file.kwargs.get('user'),