Change error on empty subjectAltNames #122

Job	Run time
Build docs & run doctests	31s
Verify dev env (ubuntu-latest)	21s
Run linters	33s
Verify dev env (windows-latest)	1m 9s
Verify dev env (macos-latest)	31s
Type-check package	19s
Tests on 3.8	57s
Tests on 3.9	1m 12s
Tests on 3.10	1m 6s
Tests on 3.11	59s
Tests on 3.12	1m 11s
Tests on pypy-3.8	1m 12s
Tests on pypy-3.9	1m 10s
Combine & check coverage	8s
required-checks-pass	3s
	11m 22s

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