[lib] Fix UpdateRelationshipMessageSpec.validator

Summary:
Based on https://github.com/CommE2E/comm/commit/f83bf3a95128fae846cb155720f6ac7851d517bd

Depends on D11455

Test Plan: Unit tests in validation.test.js cover this

Reviewers: ginsu, will, varun, tomek

Reviewed By: ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11457