Handle null accessory strings safely

In certain misconfigured car head units, ACCESSORY START UEVENT is
delivered again, while former ACCESSORY START UEVENT was already
configured. This leads to null accessory strings being passed to the
accessory mode, which expect the manufacturer and model strings to be
non-null. This causes a platform reset. This change gracefully handles
the case of expected non-null accessory strings being null by not
updating the USB Accessory for the same.

Bug: 275263867
Bug: 356421183
Flag: EXEMPT bugfix
Change-Id: I108b46ff164877d92d4e8fb6d53beca230df4fa0