btm_get_dev_class: Create empty record if not found

We may not have a device record at that time, so create one if needed.
This fixes the diff introduces in https://android-review.googlesource.com/c/platform/system/bt/+/1480894/9/stack/acl/btm_acl.cc#b2183

Bug: 159815595
Tag: #refactor
Test: compile & verify basic functions working
Test: pair to a speaker
Change-Id: I0149ecb7a11ed33f6cb6344f74c08746d948c513