Change error on empty subjectAltNames #115

Job	Run time
Build docs & run doctests	27s
Verify dev env (ubuntu-latest)	45s
Verify dev env (windows-latest)	1m 3s
Run linters	28s
Verify dev env (macos-latest)	4m 47s
Type-check package	23s
Tests on 3.8	56s
Tests on 3.9	1m 1s
Tests on 3.10	1m 5s
Tests on 3.11	1m 6s
Tests on 3.12	1m 5s
Tests on pypy-3.8	1m 9s
Tests on pypy-3.9	1m 17s
Combine & check coverage	14s
required-checks-pass	2s
	15m 48s

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