Fix failure to validate pasted links

[Nantris]

Also defaults validate option to val => !!val rather than undefined

Please describe your changes

Fix failure to validate pasted links (#5060)

How did you accomplish your changes

Add code which was missing to check pasted links are valid like all others

How have you tested your changes

I have not

How can we verify your changes

Probably manually. I assume automated tests do not cover this since this regression was able to sneak into 2.2.0

Remarks

Fixes #5060

Checklist

• The changes are not breaking the editor
• Added tests where possible
• Followed the guidelines
• Fixed linting issues

Related issues

#5060

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[Nantris]

These changes should also be backported to 2.2.x

[Nantris]

@bdbch @svenadlung ready for review when you can

[Nantris]

This change is essential to fix a breaking regression in 2.2.x and 2.3.x. Can anyone take a look?

[loui7]

LGTM