Fix for gutentags#statusline() with no args.

If I add ```set statusline+=%{gutentags#statusline()}``` to my vimrc, as suggested by the help document, I see the following error when I edit a file under my working directory:

    Error detected while processing function gutentags#statusline:
    line    9:
    E121: Undefined variable: a:1

The reason for this is the statusline() function tests that a:0 >= 0 instead of > 0 before accessing a:1. If the argument count is 0, then a:1 is undefined, of course.