Fix gimp palette files being exported with a bogus Name field

Previously, most gpl files would write an empty string Name field, like "Name: \n" and that would cause an error on import because of the empty name.

So the fix is in two parts. One of which is in this extension code:

• If the name is an empty string, don't write it out. This matches behavior elsewhere in Inkscape for writing out gpl files. (Search for the two implementations of save_gimp_palette)

And the other is in the main Inkscape code:

• Be more permissive when reading gpl files, and don't error out when the name is empty.

Related to inkscape#6234 (closed) and inkscape!8001 (merged)