FindOpenSSL: Prefer OPENSSL_ROOT_DIR only if non-empty
Since !8486 (merged) we use OPENSSL_ROOT_DIR
whenever it is
defined, even if it is an empty string. This breaks a pattern in
existing projects that define an empty OPENSSL_ROOT_DIR
cache entry so
that cmake-gui
users can fill it in. Use the value only if non-empty.
Issue: #18352 (closed)
Backport: release