Fix NPE in KeyphraseEnrollmentInfo.toString()

If there are no enrollment applications on the system, but someone still
makes a KeyphraseEnrollmentInfo and tries to print it, it would generate
a NPE on a map object. Instead of setting the map to null when we don't
find any enrollment applications, we can just set it to an empty map.

Bug:28622866
Change-Id: I023e6fd90effd3143c19817a0d6637a013bebc31