Set-AMTDevice removes tags if parameter is not specified

[michaelmumenthaler]

Issue

When using the Set-AMTDevice command without specifying the -Tags parameter all tags are removed from the device.

Steps to reproduce

PS> Set-AMTDevice Device01 -Tags "London","Store12"

PS> Set-AMTDevice Device01 -Name DeviceNewName

PS> Get-AMTDevice DeviceNewName

Expected behaviour

The tags should only be removed if the -Tags parameter is set with the value $null or "".