Fix possible IllegalArgumentException during a database migration.

src/org/thoughtcrime/securesms/database/helpers/SQLCipherOpenHelper.java

@@ -387,6 +387,7 @@ public class SQLCipherOpenHelper extends SQLiteOpenHelper {
       if (oldVersion < SELF_ATTACHMENT_CLEANUP) {
         String localNumber = TextSecurePreferences.getLocalNumber(context);
 
+        if (!TextUtils.isEmpty(localNumber)) {
           try (Cursor threadCursor = db.rawQuery("SELECT _id FROM thread WHERE recipient_ids = ?", new String[]{ localNumber })) {
             if (threadCursor != null && threadCursor.moveToFirst()) {
               long          threadId     = threadCursor.getLong(0);
@@ -399,6 +400,7 @@ public class SQLCipherOpenHelper extends SQLiteOpenHelper {
             }
           }
         }
+      }
 
       db.setTransactionSuccessful();
     } finally {