Do not collect empty build props

So that build prop is collected only from non-empty property modules in
recovery_build_prop.

Test: CI && ABTD
Bug: 381888358
Change-Id: I885b89b0dd0baa67036b4ae5a134b033471377c3
diff --git a/android/recovery_build_prop.go b/android/recovery_build_prop.go
index 91d1904..ac7d2ec 100644
--- a/android/recovery_build_prop.go
+++ b/android/recovery_build_prop.go
@@ -83,8 +83,10 @@
 		r.properties.Product_build_prop,
 		r.properties.System_ext_build_prop,
 	} {
-		if buildPropPath := PathForModuleSrc(ctx, proptools.String(buildProp)); buildPropPath != nil {
-			buildProps = append(buildProps, buildPropPath)
+		if buildProp != nil {
+			if buildPropPath := PathForModuleSrc(ctx, proptools.String(buildProp)); buildPropPath != nil {
+				buildProps = append(buildProps, buildPropPath)
+			}
 		}
 	}
 	return buildProps