new password shouldn't be empty string

enter "Home screen backup password" in "Developer Settings",
do nothings and then click "Set backup password", There will
be a toast display "New backup password set".
The new password field cannot be empty.

Bug: https://partnerissuetracker.corp.google.com/u/2/issues/111261338
Test: empty string can not be set as a password

Signed-off-by: xiamin <xiamin@xiaomi.com>

src/com/android/settings/SetFullBackupPassword.java

@@ -21,6 +21,7 @@ import android.app.backup.IBackupManager;
 import android.os.Bundle;
 import android.os.RemoteException;
 import android.os.ServiceManager;
+import android.text.TextUtils;
 import android.util.Log;
 import android.view.View;
 import android.view.View.OnClickListener;
@@ -96,6 +97,11 @@ Log.i(TAG, "failure; password mismatch?");
     }
 
     private boolean setBackupPassword(String currentPw, String newPw) {
+        // new password can't be empty
+        if (TextUtils.isEmpty(newPw)) {
+            return false;
+        }
+
         try {
             return mBackupManager.setBackupPassword(currentPw, newPw);
         } catch (RemoteException e) {