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
diff --git a/apex/apex.go b/apex/apex.go
index 7a2a04a..64e1760 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -2523,8 +2523,10 @@
 		}
 
 		// Prebuilts
-		prebuiltsLabelList := android.BazelLabelForModuleDeps(ctx, overridableProperties.Prebuilts)
-		attrs.Prebuilts = bazel.MakeLabelListAttribute(prebuiltsLabelList)
+		if overridableProperties.Prebuilts != nil {
+			prebuiltsLabelList := android.BazelLabelForModuleDeps(ctx, overridableProperties.Prebuilts)
+			attrs.Prebuilts = bazel.MakeLabelListAttribute(prebuiltsLabelList)
+		}
 
 		// Compressible
 		if overridableProperties.Compressible != nil {