fix: at_cli_commons: handle malformed atsigns (no leading `@`) in CLIBase constructor #1202

Job	Run time
functional_tests_at_onboarding_cli (at_onboarding_cli_functional_tests)	2m 24s
build_and_test (at_lookup)	2m 20s
build_and_test (at_chops)	47s
build_and_test (at_onboarding_cli)	43s
build (at_contact)	34s
build (at_server_status)	45s
build_and_test (at_commons)	41s
build_and_test (at_utils)	2m 12s
	10m 26s

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