Menu link field required setting is ineffective

Also adding additional tweaks:

Makes menu title and menu select required when field is required.

Tweaks for field-level Default menu for new links so a new '- None -' option is available. When the field is required, this effectively forces the user to make a conscious decision for which menu the link is placed in. I've needed to adjust the fallback behaviour to not use 'main' when the Default menu for new links value is empty. I think this existing/old behaviour was a safety fallback anyway.