Change error on empty subjectAltNames #129

Job	Run time
Build docs & run doctests	22s
Run linters	31s
Verify dev env (ubuntu-latest)	22s
Type-check package	23s
Verify dev env (windows-latest)	1m 9s
Tests on 3.8	1m 6s
Verify dev env (macos-latest)	24s
Tests on 3.9	1m 5s
Tests on 3.10	1m 3s
Tests on 3.11	58s
Tests on 3.12	1m 12s
Tests on pypy-3.8	1m 10s
Tests on pypy-3.9	1m 15s
Combine & check coverage	13s
required-checks-pass	3s
	11m 16s

Provide feedback