tls: do not crash on STARTTLS when OCSP requested

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

tls

TLSSocket should not have a hard dependency on tls.Server, since it may be running without it in cases like STARTTLS.

Fix: #10704 (closed)