fix: guard against undefined in drop mempool msgs #3103

Job	Run time
lint	1m 6s
test-snp	1m 29s
test-rosetta (rosetta-construction)	2m 22s
test-rosetta (rosetta-cli-construction)	2m 23s
test-rosetta (rosetta)	3m 21s
test-rosetta (rosetta-cli-data)	2m 10s
test (rpc)	1m 38s
test (event-replay)	1m 50s
test (bns)	3m 9s
test (btc-faucet)	1m 34s
test (api)	10m 13s
test (bns-e2e)	7m 5s
test-2_5 (pox-4-stack-extend-increase)	2m 9s
test-2_5 (pox-4-burnchain-stack-stx)	1m 34s
test-2_5 (faucet-stx)	1m 35s
test-2_5 (pox-4-rosetta-btc-addr-types)	4m 58s
test-2_5 (pox-4-rosetta-cycle-phases)	1m 29s
test-2_5 (pox-4-btc-address-formats)	2m 18s
test-2_5 (pox-4-delegate-revoked-stacking)	2m 11s
test-2_5 (pox-4-rosetta-segwit)	1m 47s
test-2_5 (pox-4-delegate-stacking)	1m 40s
test-2_5 (pox-4-delegate-aggregation)	2m 7s
test-2_5 (block-zero-handling)	1m 22s
test-2_5 (pox-4-burnchain-delegate-stx)	1m 43s
semantic-release	28s
build-publish	1m 45s
	1h 5m 26s

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