Format ListPreference summary with empty string if entry is null

Adding serif-monospace and sans-serif-monospace font families bumped
out the previous monospace, which left the summary as %s. Returning
an unformatted summary is never correct, and we should simply return
a summary populated with an empty string.

BUG: 17666124
Change-Id: Ic22dc5dc87d9ba3ad6f6b92ee059f5c98cbf103a