Fix config URL so that the phones don't get an invalid URL when $domain_name is unset. They would previously receive `/app/provision` with no domain/ip included, which makes the phone unable to provision again once they have been added to the Devices page without manually re-setting the config URL or factory defaulting the device.
The alternative solution is that the PHP code could always ensure that the $domain_name variable is set even if the device doesn't exist. My suggestion is to see if it can be set to the same domain/IP as the original provision request from this unconfigured device
Excludes the P8352 value if the default setting grandstream_speaker_ring_volume is unset. Depends on PR #5522
Before anyone asks, Mandatory just means that the variable must be valid, not that it must be included in every XML download. The phone already has a valid value in its internal settings, so as long as we stay safe from setting it to an empty value or outside of the range of 0-7, we are safe. Excluding it is a valid option, which is also true for all settings labelled Mandatory in the Grandstream templates.