Fix #989 - treat empty pin specification like NO_PIN #94

Job	Run time
tests (ubuntu-latest, tests)	48s
tests (macos-latest, tests)	1m 28s
tests (windows-latest, tests_nosan)	2m 16s
build (ubuntu-latest, noradio)	2m 47s
build (ubuntu-latest, wifi)	3m 6s
build (ubuntu-latest, bt)	2m 32s
build (macos-latest, noradio)	2m 53s
build (macos-latest, wifi)	2m 58s
build (macos-latest, bt)	2m 57s
build (windows-latest, noradio)	5m 24s
build (windows-latest, wifi)	5m 27s
build (windows-latest, bt)	7m 2s
	39m 38s

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