fix(registrar): a client that does not use '+sip.instance' could have...

Reviewer guidelines

The following requirements must be met before merging:

flexisip	done	N.A.	Tests added
	done	N.A.	Related MR pipelines succeeded
	done	N.A.	SHA1 consistency of cherry-picked commits
linphone-sdk	done	N.A.	On a tag... (Except for master, which is allowed to use yet-to-be-released versions of the SDK)
	done	N.A.	...From the corresponding branch
docs	done	N.A.	CHANGELOG up to date
	done	N.A.	wiki (public documentation) up to date

Closes FLEXISIP-402