Change error on empty subjectAltNames #124

Job	Run time
Build docs & run doctests	32s
Verify dev env (ubuntu-latest)	18s
Run linters	40s
Verify dev env (windows-latest)	45s
Type-check package	21s
Verify dev env (macos-latest)	30s
Tests on 3.8	1m 10s
Tests on 3.9	1m 8s
Tests on 3.10	1m 3s
Tests on 3.11	59s
Tests on 3.12	1m 11s
Tests on pypy-3.8	1m 12s
Tests on pypy-3.9	1m 23s
Combine & check coverage	20s
required-checks-pass	2s
	11m 34s

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