FindOpenSSL: Prefer OPENSSL_ROOT_DIR only if non-empty

Since commit 5cc8a69867 (FindOpenSSL: Trust the user's OPENSSL_ROOT_DIR,
2023-05-17, v3.27.0-rc1~71^2) 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