Add gettls null backend test #3207

Job	Run time
lint-receptor	25s
container	2m 9s
receptor (Go 1.20)	3m 30s
receptor (Go 1.21)	3m 35s
Lint receptorctl / Run `receptorctl` check_format session	17s
Lint receptorctl / Run `receptorctl` check_style session	23s
Receptorctl test coverage (coverage)	1m 27s
Run receptorctl tests / Run `receptorctl` tests-3.10 session	1m 17s
Run receptorctl tests / Run `receptorctl` tests-3.8 session	1m 20s
Run receptorctl tests / Run `receptorctl` tests-3.11 session	1m 13s
Run receptorctl tests / Run `receptorctl` tests-3.12 session	1m 16s
Run receptorctl tests / Run `receptorctl` tests-3.9 session	1m 10s
	18m 2s

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