Fix NPE with Roboletric by returning emptyArray

Previous change I69e21940eaec4427f62abe59ae003b4f60855f22
did not handle the case of missing /system/etc/fonts.xml
This happens on roboletric test and crash with NPE.

Bug: 112843740
Test: N/A
Change-Id: I6d183dab9af66abf777f2e5b1fa633ff979fea55