Themes: uninstallThemeForAllApps shouldn't check if map is empty if it's null

Potential NPE introduced in ec2200878706735cdd2caf4d934d988296998e85

Change-Id: I6d1ec165957c2be63086a18af2236e46978e9c8a
(cherry picked from commit 59a9fc5c1d6164af5856725ca0d494ce70e3ae4d)