Support ACM certs without CN

[charhate]

Problem

import_certificate raises IndexError: list index out of range when the certificate has no Common Name (CN) in its subject. The bug is in acm/models.py:

Modern certificates commonly omit CN and rely solely on Subject Alternative Names (SANs). Real AWS ACM accepts such certificates without issue.

Fix

• CN is parsed defensively; falls back to the first SAN when absent (matching real AWS ACM DomainName behaviour), or "" if there are no SANs either
• Issuer CN is also parsed defensively
• SANs are parsed once in init into self.sans and reused in describe(), removing the duplicate SAN-parsing block that was there before
• domain_names in describe() guards against adding an empty string when both CN and SANs are absent