Support empty srcs for configurable attrs

Test: mixed build linkerconfig
Change-Id: I1ee56f7fa46ce2971ecae6d2ca9258631a32f46a
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index 388c8cf..6bdfc0e 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -445,7 +445,7 @@
 		return prettyPrint(propertyValue.Elem(), indent)
 	case reflect.Slice:
 		if propertyValue.Len() == 0 {
-			return "", nil
+			return "[]", nil
 		}
 
 		if propertyValue.Len() == 1 {