GD-HCI: Write empty value to storage instead of nullptr

* GD storage layer forbids writing nullptr
* Fix the instance in legacy stack that writes a nullptr when it
  wants to write an empty value

Bug: 164979578
Tag: #gd-refactor
Test: pair with hearing aid devices
Change-Id: I8280c66f6862f92a9fbcb87b1a1216d59ad2983d