fix: var 'message' = type 'str' in 'new_git_tag()'
    
    If 'message' parameter is provided, pygit2 requires the value to be a string.
    Former default to 'None' caused two tests to fail with a 'TypeError'.
    Adjusted to enforce that empty string get passed to pygit2 if 'message' is None.
    
    https://github.com/libgit2/pygit2/blob/master/src/repository.c#L1225