detect: fix auto-detection of metric exporters to handle none correctly #7156

Job	Run time
prepare	5s
archutil-arm64	0s
validate (lint-default)	11m 21s
validate (lint-labs)	11m 4s
validate (lint-nydus)	11m 16s
validate (lint-proto)	32s
validate (lint-yaml)	30s
validate (validate-archutil)	32s
validate (validate-docs)	43s
validate (validate-doctoc)	21s
validate (validate-generated-files)	40s
validate (validate-shfmt)	14s
validate (validate-vendor)	31s
	37m 49s

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