Objects dialog and Objects Properties dialog should both allow labels to be blank or unset

Split from inbox#369, which was migrated from: https://bugs.launchpad.net/inkscape/+bug/1811607

Steps to reproduce:

• Open Inkscape
• Create a couple objects (e.g. Rectangles)
• Open Objects Dialog and Object Properties Dialog
• Try to make a label blank in the objects dialog (fails)
• Set a label in Object Properties, then set it to blank (so no characters).
• Note that the objects dialog now gives a blank label.

What happened?

• A blank label is not allowed in Objects dialog but is in Object Properties.

What should have happened?

• Consistent behavior in both dialogs
• I'd prefer for the label to be unset, but a conservative choice is to set inkscape:label="" when it is blank (so when a user sets a blank label in the Objects label, it doesn't fallback to the id).

Inkscape Version and Operating System:

• Inkscape 1.0alpha (2bfff771, 2019-04-16) (closest parent from master being 69e4b63d, 2019-04-15) Linux Mint 19.1
• Inkscape 0.92.4 (5da689c313, 2019-01-14) windows (OP)
• Inkscape 1.0alpha (4855a8a2) (Qantas94heavy)
• Inkscape 1.1-dev (c990a67c, 2020-04-09) (jhofinger in issue this was split from)
• Inkscape 1.1-dev (838ae0f2, 2020-07-14, custom) Linux Mint 19.1 (confirming it's still happening)