core: remove redundant null check

This patch removes a redundant null check in BrandingTheme.
The getResourceBundle().getString(...) method is called. This method
doesn't return null. If the key doesn't exist an exception is thrown,
but null is never returned. Thus, the check is redundant. Removing it.

Change-Id: Iea4cda8debba202683b4048d9da5ccb86f2ca0d1
Signed-off-by: Oved Ourfali <oourfali@redhat.com>