Regression: Allowing NULL as the audio spatializer is disallowing selection in the config dialog

[david-c-kline]

The recent change to allow null for the audio spatializer has resulted in disallowing the setting to be changed in the configuration dialog.

I propose that we never disallow changing the spatializer, even if the setting is acceptable.

[david-c-kline]

This is starting to look like a side effect of not having any spatializers installed (default in Unity 2019.3)

[david-c-kline]

Confirmed that after installing a Unity package containing a spatializer, the dialog behaves as expected