Change error on empty subjectAltNames #127

Job	Run time
Build docs & run doctests	24s
Run linters	39s
Type-check package	18s
Tests on 3.8	58s
Tests on 3.9	1m 0s
Tests on 3.10	1m 4s
Verify dev env (ubuntu-latest)	22s
Tests on 3.11	59s
Verify dev env (windows-latest)	1m 0s
Tests on 3.12	1m 6s
Verify dev env (macos-latest)	26s
Tests on pypy-3.8	1m 17s
Tests on pypy-3.9	1m 15s
Combine & check coverage	14s
required-checks-pass	2s
	11m 4s

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