Pass nil instead of []string{} to ctx.Glob* functions

[]string{} is unnecessary, just use nil.

Test: m ALLOW_MISSING_DEPENDENCIES=true
Change-Id: Ia9aeb2ffc483429787da0e473a7f1bc87eb4cad1
diff --git a/android/module.go b/android/module.go
index fe6c0de..07c4e8f 100644
--- a/android/module.go
+++ b/android/module.go
@@ -1177,7 +1177,10 @@
 func (ctx *androidModuleContext) ExpandSourcesSubDir(srcFiles, excludes []string, subDir string) Paths {
 	prefix := PathForModuleSrc(ctx).String()
 
-	expandedExcludes := make([]string, 0, len(excludes))
+	var expandedExcludes []string
+	if excludes != nil {
+		expandedExcludes = make([]string, 0, len(excludes))
+	}
 
 	for _, e := range excludes {
 		if m := SrcIsModule(e); m != "" {