a_tty_bind_parse(): trim non-editable bindings once again, reject empty ones..

a_tty_bind_parse(): trim non-editable bindings once again, reject empty ones..

The manual states that empty key bindings are rejected, but by
specifiying a non-empty shell-token this yet has been waved
through, for example

  bind base a ' '

Since that effectively results in invoking the default command
this can be seen as an error, whereas it is ok for the editable

  bind base a ' '@

So, in the non-editable case, trim the resulting string once
again, so that it will be seen as a truly empty sequence, if it is.