Check null before passing to Integer.parseInt

Just after user data is cleared, user preference associated with
PREF_KEYBOARD_LAYOUT is always empty. In such case, we might
want to return the default value immediately, rather than
calling Integer.parseInt(null) and catching NumberFormatException
unnecessarily.

BUG: 13472379
Change-Id: I8dca9a22780d057013c032bd1f56e730dcada3ce