Only override prebuilts property if the override_apex.prebuilts is

explicitly specified (not nil).

This allows the base apex's prebuilts to be surfaced to the override
apex target.

Discovered when com.google.android.adbd's prebuilts didn't contain its
init.rc module.

Bug: 222588072
Test: CI
Change-Id: I925e98d53f7d9208652c629799ab568521ba7dfe