Update FilamentShield.php

[mahdimirhendi]

Fix for Issue #471:

•	Added a check to ensure that the getHeading method returns a non-null value before using it.
•	Removed redundant check for TableWidget in the heading validation logic, as it’s already handled in the existing conditions.

This should address the issue with heading retrieval in widgets

[what-the-diff]

PR Summary

• Improved Widget's Heading Verification Process
  The way we determine the heading of our widgets has been updated. Instead of the older and complex method, we now use a simple private method - hasValidHeading($widgetInstance) - which makes our code cleaner and easier to maintain.

• Addition of a New Method
  A new method named hasValidHeading($widgetInstance) is introduced. This function checks whether the widget we're dealing with is indeed a Widget. It also validates that it has a method called getHeading and the heading itself is not empty. This will ensure that every widget used in our system complies with these rules, avoiding errors down the line.