add @Nullable to warn via IDE

Signed-off-by: tobiasKaminsky <tobias@kaminsky.me>

src/main/java/com/owncloud/android/datamodel/FileDataStorageManager.java

@@ -120,7 +120,8 @@ public class FileDataStorageManager {
     }
 
 
-    public OCFile getFileById(long id) {
+    public @Nullable
+    OCFile getFileById(long id) {
         Cursor c = getFileCursorForValue(ProviderTableMeta._ID, String.valueOf(id));
         OCFile file = null;
         if (c.moveToFirst()) {