AppCenter crashes when open an empty category

### What Happened?

Some categories are empty (or appear to be) for e.g. `Education`, `Office` and `System` and opening them causes the store to close with segmentation fault

### Steps to Reproduce

1. Open AppCenter
2. Choose one of these: `Education`, `Office` or `System`

### Expected Behavior

Show "No Apps" screen

### OS Version

7.x (Horus)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell
GDB Log:

Thread 1 "io.elementary.appcenter" received signal SIGSEGEV, Segmentation fault

0x00007ffff70249fb in as_release_get_timestamp() from /lib/x86_64-linux-gnu/libappstream.so.4
```

Normal Log:

```
Core Dumped
```