Change error on empty subjectAltNames #119

Job	Run time
Run linters	5s
Build docs & run doctests	28s
Type-check package	23s
Tests on 3.8	1m 3s
Tests on 3.9	1m 5s
Verify dev env (ubuntu-latest)	23s
Tests on 3.10	59s
Verify dev env (windows-latest)	59s
Tests on 3.11	59s
Verify dev env (macos-latest)	2m 15s
Tests on 3.12	1m 7s
Tests on pypy-3.8	1m 10s
Tests on pypy-3.9	1m 14s
Combine & check coverage	9s
required-checks-pass	3s
	12m 22s

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