Not possible to set user's properties to the null value using Set-EntraUser

For example, `Set-EntraUser -ObjectId $diegoId -AgeGroup $null` fails.

`$null` is converted to:

```
{
  "ageGroup": ""
}
```

instead of:

```
{
  "ageGroup": null
}
```

I assume we will have the same problem with other Entra commands.