Fix: Error when scss file is empty #327 (#340)

* Fix: Error when scss file is empty #327

* Return empty string when code is empty

* Handling empty files

* No need to load in mightHaveDependencies()

* mightHaveDependencies doesn't run if this.contents is empty.

src/Asset.js

@@ -51,7 +51,7 @@ class Asset {
   async getDependencies() {
     await this.loadIfNeeded();
 
-    if (this.mightHaveDependencies()) {
+    if (this.contents && this.mightHaveDependencies()) {
       await this.parseIfNeeded();
       this.collectDependencies();
     }