Fix: Handle null event.submitter in enhanced form submission

[shethaadit]

Fix: Handle null event.submitter in enhanced form submission

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Description

• This PR fixes an issue where calling requestSubmit on a form with the data-enhance attribute caused an uncaught TypeError due to accessing a member on a null event.submitter.
• The change replaces the non-null assertion operator (!) with the optional chaining operator (?) to properly handle cases where event.submitter is null.

By addressing this, the form navigation is enhanced without throwing errors, ensuring smoother functionality and user experience.

Fixes #58244

[shethaadit]

Hi @mkArtakMSFT/team, could you please help review the PR and check test case failure?