fix: error ... executing doInBackground() #138 (#139)

As requested in ST 1341351, this PR replaces the throw if `files` is null.

android/widgets/src/main/java/org/nativescript/widgets/Image/DiskLruCache.java

@@ -254,7 +254,7 @@ public final class DiskLruCache implements Closeable {
     public static void deleteContents(File dir) throws IOException {
         File[] files = dir.listFiles();
         if (files == null) {
-            throw new IllegalArgumentException("not a directory: " + dir);
+            return;
         }
         for (File file : files) {
             if (file.isDirectory()) {