[REVIEW] fix for fdo#46816, don't show error message for empty range name in Define Names

[Markus Mohrhard]

Hey,

[1] improves the dialog a bit. The error message is a bit misleading
and we can just use the disabled Add button as sign that you can't add
an empty range name.

It would be nice if this could be cherry-picked to 3-5 and maybe 3-5-1.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=6e93e78c85e0c4eea174483f1d27ee38fd00ee21