AXI-MM Mem Ctrl Write response fix: fixed pop from empty std::queue representing fifo issue, causing the b valid to be forced high

[StephenMoreOSU]

The issue comes from the bvalid signal being set if the output write queue is not empty and somewhere along the line an empty std::queue was popped, underflowing "_output_write_queue_occupancy" and making the mem controller think that there were memory responses to be sent, forcing bvalid high.

Below is a trace printout demonstrating the issue with the previous AXI-MM B resp signals coming from memory controller

Sent AW Trans id 0 @ Cycle 5!
black_box_0_inst: Sent W Transaction id: 0 @ Cycle 7!
[MEM CTRL] WRITE CALLBACK @ ch 0 @ addr [0] data_word: 0
[MEM CTRL] Sending back B Response! @ Cycle 1d!
[MEM CTRL] _output_write_queue_occupancy: 1
[MEM CTRL] Sending back B Response! @ Cycle 1e!
[MEM CTRL] _output_write_queue_occupancy: 1
[MEM CTRL] Sending back B Response! @ Cycle 21!
[MEM CTRL] _output_write_queue_occupancy: ffffffff
[MEM CTRL] Sending back B Response! @ Cycle 23!
[MEM CTRL] _output_write_queue_occupancy: fffffffe