Blank editable legend item fixes

[alexcjohnson]

Fixes #2576

• In editable mode, pads all blank or short legend items out to the length of the longest one (or 4 characters at least) - note this is the character length, not the svg text length which will usually be shorter.
• groupby and regular traces work the same now - deleting the value completely leaves it blank, rather than reverting to the default like groupby had been doing.

cc @etpinard

[etpinard]

Looks good 💃