files: avoid an abortion when excuting a command in a new buffer

The key sequence ^R ^X M-F <Enter> would cause nano to abort, because
it would try to add an empty string to a history list.  (Furthermore,
simply don't execute an empty command, because it is pointless.)

Reported-by: Marco Diego Aurélio Mesquita <marcodiegomesquita@gmail.com>
1 file changed