Don't break existing expiry time of "infinity" when modifying protection

Currently, modifying protection with infinite expiry time causes it to
be set to a blank "Other time", which causes an error. This fixes it to
act the same way as existing expiry times.

Bug: 70062
Change-Id: I3a7d45a677151597a941ecb11584bc1d67001136
Merge "Don't break existing expiry time of "infinity" when modifying protection"