Change error on empty subjectAltNames #126

Job	Run time
Run linters	40s
Type-check package	20s
Verify dev env (ubuntu-latest)	27s
Tests on 3.8	1m 4s
Verify dev env (windows-latest)	46s
Verify dev env (macos-latest)	2m 18s
Tests on 3.9	1m 1s
Tests on 3.10	58s
Tests on 3.11	56s
Tests on 3.12	1m 12s
Build docs & run doctests	22s
Tests on pypy-3.8	1m 19s
Tests on pypy-3.9	1m 14s
Combine & check coverage	10s
required-checks-pass	2s
	12m 49s

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