Reviewer Ellie: Don't push IRSs to homestead_cache unless they contain a SIP URI

Ellie, please can you review this change to stop us from pushing Implicit Registration Sets that don't contain a SIP URI to homestead_cache.

If you have any questions or suggestions of what I should have done better, I'm happy to talk at great length as to why I've done what I've done.

Also, we currently have no UTs for the code I've changed. I haven't added any. I've live tested this pretty extensively though.