[BUG]: Empty the Front template and click save, no warning

### Checked for duplicates?

- [X] This issue is not a duplicate

### Does it also happen in the desktop version?

- [X] This bug does not occur in the latest version of Anki

### What are the steps to reproduce this bug?

If I set the Front template empty, no warning shows up.
However, on the desktop version of Anki, it will show a warning if you want to empty the front and don't allow the user to empty the front.
I think we should do the same things that don't allow users to empty the front, the same as the desktop version of Anki

Steps to Reproduce this bug:
1. click card brower
2. click to edit one card
3. click Cards to edit the template
4. clear the text in front template
5. click save

### Expected behaviour

It should not allow the user to empty the Front, just as the desktop Anki does.

### Actual behaviour

It allow the user to empty front template and save.
If you review this card, it will display like this:

### Debug info

```text
AnkiDroid Version = 2.17.6 (568538896be943818ca793dd2635822265712def)

Backend Version = 0.1.34-anki23.12.1 (23.12.1 1a1d4d5419c6b57ef3baf99c9d2d9cf85d36ae0a)

Android Version = 10 (SDK 29)

ProductFlavor = amazon

Manufacturer = unknown

Model = Android SDK built for x86

Hardware = ranchu

Webview User Agent = Mozilla/5.0 (Linux; Android 10; Android SDK built for x86 Build/QSR1.210820.001; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/74.0.3729.185 Mobile Safari/537.36

ACRA UUID = b8c60fbd-e11e-4dec-9008-73f0d38b53dd

Crash Reports Enabled = true
```

### (Optional) Anything else you want to share?

_No response_

### Research

- [X] I am reporting a bug specific to AnkiDroid (Android app)
- [X] I have checked the [manual](https://ankidroid.org/docs/manual.html) and the [FAQ](https://github.com/ankidroid/Anki-Android/wiki/FAQ) and could not find a solution to my issue
- [X] (Optional) I have confirmed the issue is not resolved in the latest alpha release ([instructions](https://docs.ankidroid.org/manual.html#betaTesting))