Problem: in autocmd APIs, a non-nil "pattern" containing only empty
'sub'-patterns is silently treated as nil, causing the fallback value to be
unexpectedly used instead.
Solution: for nvim_create_autocmd(), raise a validation error (as no autocmds
would be created). For nvim_{exec,clear}_autocmds(), make it a no-op (as
matching no autocmds is not an error).