Fix nullptr access in some functions of BufferD3D12.h

Steps to reproduce:

1. Have invalid entry, so that GetVersionInternal() return null
2. Call a method which uses the result (i.e., GetGPUVirtualAddress in DrawBuffersCommon)
3. Crash

Actual results: Engine crashes

Expected results: No crash, just skipping a draw call with logging

Reproducible with versions: 6000.x

Not reproducible with versions: -

Can’t test with versions: -

Tested on (OS): Windows 11