GetConfig() returns ("", nil) for missing/empty config keys.
getRepoConfig() then tried json.Unmarshal([]byte(""), ...) which
fails with "unexpected end of JSON input".
Add empty value check before JSON parsing - return empty map when
config value is empty string.
Closes GH#680
Co-Authored-By: dylan-conlin <dylan-conlin@users.noreply.github.com>