Handle the Trace32 read memory command when the memory type is unknown

[alexsmendez]

This handles the Trace32 GDB server issue reported by Pedro Texeira (tracked by ADO ticket:49235111).

• This PR fixes the function ExdiGdbSrv!Trace32GdbServerMemoryHelpers::GetGdbSrvReadMemoryCmd() to always return a command memory string value (rather than nullptr) when the memory type command parameter has not been determined (it default to VA type). -The function now returns the "generic GDB memory access command" for the Trace 32 GDB server that supports customized GDB memory commands depending on the memory type parameter requested.