code: public Optional<Variable> getOptionalVariable(Object groupIdOrPath, String key) {
          try {
              return (Optional.ofNullable(getVariable(groupIdOrPath, key)));
          } catch (GitLabApiException glae) {
              return (GitLabApi.createOptionalFromException(glae));
          }
      }
docs: Get the details of a group variable as an Optional instance.
  
  <pre><code>GitLab Endpoint: GET /groups/:id/variables/:key</code></pre>
  
  @param groupIdOrPath the group ID, path of the group, or a Group instance holding the group ID or path
  @param key the key of an existing variable, required
  @return the Variable for the specified group variable as an Optional instance