Problem: When 'signcolumn' is set to `number` but a line has a sign
without text, the line number disappears (finite-state-machine)
Solution: Verify that a sign actually contains text before rendering the
line number (glepnir)
fixes: vim/vim#17169closes: vim/vim#172821b186833c1
Co-authored-by: glepnir <glephunter@gmail.com>