Do not call get_user_id if it is not needed

On systems without any environment variables and no pwd module, gitpython crashes as it tries to read the environment variable before looking at its config.

Author: eric-wieser

git/util.py

@@ -704,7 +704,11 @@ def default_name() -> str:
                 setattr(actor, attr, val)
             except KeyError:
                 if config_reader is not None:
-                    setattr(actor, attr, config_reader.get_value('user', cvar, default()))
+                    try:
+                        val = config_reader.get_value('user', cvar)
+                    except Exception:
+                        val = default()
+                    setattr(actor, attr, val)
                 # END config-reader handling
                 if not getattr(actor, attr):
                     setattr(actor, attr, default())