Change error on empty subjectAltNames #128

Job	Run time
Build docs & run doctests	23s
Run linters	38s
Verify dev env (ubuntu-latest)	23s
Verify dev env (windows-latest)	59s
Type-check package	23s
Tests on 3.8	1m 1s
Verify dev env (macos-latest)	28s
Tests on 3.9	1m 4s
Tests on 3.10	1m 3s
Tests on 3.11	59s
Tests on 3.12	1m 40s
Tests on pypy-3.8	1m 8s
Tests on pypy-3.9	1m 8s
Combine & check coverage	13s
required-checks-pass	2s
	11m 32s

Provide feedback