FindOpenSSL: Prefer OPENSSL_ROOT_DIR only if non-empty

Brad Kingrequested to merge
brad.king/cmake:FindOpenSSL-trust-root-dir into master

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

Merge request reports