Refactor GitLabConnectionProperty code

[shivajee98]

Replace explicit cast and null check with Objects.requireNonNull() for improved robustness and readability.

• Replaced: GitLabConnectionConfig descriptor = (GitLabConnectionConfig) Jenkins.getInstance().getDescriptor(GitLabConnectionConfig.class);
• With: GitLabConnectionConfig descriptor = Objects.requireNonNull(Jenkins.getInstance()).getDescriptor(GitLabConnectionConfig.class);

This change enhances the code by providing a more explicit null check, ensuring that the descriptor variable is guaranteed to be non-null. The use of Objects.requireNonNull() makes the code clearer and more modern.