fix fallthrough setting of localized metadata entries

Java's Map.get() returns null if there is no match, so this was always
setting each entry to whatever value was in the highest priority
locale, whether it had contents or what null.  Now, this will fall
through the priority list of locales until it finds actually contents.

app/src/main/java/org/fdroid/fdroid/data/App.java

@@ -433,7 +433,10 @@ public class App extends ValueObject implements Comparable<App>, Parcelable {
         try {
             for (String locale : locales) {
                 if (localized.containsKey(locale)) {
-                    return (String) localized.get(locale).get(key);
+                    String value = (String) localized.get(locale).get(key);
+                    if (value != null) {
+                        return value;
+                    }
                 }
             }
         } catch (ClassCastException e) {