RecoveryPointExpiryTime values are mostly blank in results from Get-AzRecoveryServicesBackupRecoveryPoint

### Description

When listing all restore points for a VM using the cmdlet Get-AzRecoveryServicesBackupRecoveryPoint, most points have an empty value in the RecoveryPointExpiryTime field, even though the expected expiry times are shown for all restore points when viewing in the portal.

Output from script:

View from portal:

### Issue script & Debug output

```PowerShell
$vault = Get-AzRecoveryServicesVault -Name $vaultName -ResourceGroupName $vaultRG
$container = Get-AzRecoveryServicesBackupContainer -ContainerType AzureVM -VaultId $vault.ID -FriendlyName $serverName -ResourceGroupName $serverRG
$item = Get-AzRecoveryServicesBackupItem -WorkloadType AzureVM -Container $container -VaultId $vault.ID
$restorePoints = Get-AzRecoveryServicesBackupRecoveryPoint -Item $item -StartDate $startDate -EndDate $endDate -VaultId $vault.ID
$restorePoints | select RecoveryPointId, RecoveryPointType, RecoveryPointTime, RecoveryPointExpiryTime, ContainerName, ContainerType | Format-Table -AutoSize
```

### Environment data

```PowerShell
Name Value
---- -----
PSVersion 5.1.22621.6133
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.22621.6133
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
```

### Module versions

```PowerShell
ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 5.3.0 Az.Accounts {Add-AzEnvironment, Clear-AzConfig, Clear-AzContext, Clear-AzDefault...}
Script 7.8.0 Az.RecoveryServices {Add-AzRecoveryServicesAsrReplicationProtectedItemDisk, Backup-AzRecoveryServicesBackupItem, Copy-AzRecoveryServicesVault, Disable-AzRecoveryServicesBackupAutoProtection...}
```

### Error output

```PowerShell

```