Does not validate 'register' field links

[gidsg]

Context

A field of a non-curie datatype may link using the 'register' field on that field definition e.g. https://field.register.gov.uk/record/register

What did you do? (steps to reproduce)

Add a new record in statistical-geography populating the register field with an invalid link value e.g. foo

What happened? (actual results)

User is allowed to continue and attempt to post to ORJ

What should have happened? (expected results)

Should fail validation

CC @Maidment64