null-check before derefing title in MenuItemImpl

The toString() method in MenuItemImpl returns mTitle.toString(),
which crashes when the title is null (which it can be, since there
is no requirement that a title be non-null, and you can get one
by simply not assigning a title to begin with or by setting it
to null).

Issue #13420311 MenuItemImpl can't handle a null title

Change-Id: I701d1d565f1d254ffdd41ca64c1abaf2906edb79