fix(FormBuilder): correct radio checked state for falsy values (fixes #650)

[vedant-jayale]

What does this PR do?

Fixes incorrect handling of radio inputs with falsy values (0, false, empty string) by replacing !$request with is_null($request) in FormBuilder.php.

Fixes #650

Why is this needed?

• Bug: Radio buttons with value="0" showed as unchecked despite being selected
• Root Cause: !$request evaluated all falsy values (0/false/"") as "missing"
• Solution: is_null() specifically detects only truly missing fields (null)

Changes Made

• Replaced !$request with is_null($request) in getRadioCheckedState()
• Maintains backward compatibility (no breaking changes)
• Aligns with standard HTML form behavior

Test Case | Before | After
value="0" | ❌ | ✅
value="1" | ✅ | ✅
Field not submitted | ✅ | ✅
  |

Fixes #650

[Imran-imtiaz48]

This update in the getRadioCheckedState method is a smart fix that addresses an important edge case—previously, the use of !$request could incorrectly treat valid falsy values like 0 or false as missing input. By switching to is_null($request), the logic now correctly handles those values, ensuring that radio buttons reflect the proper checked state. The code remains clean and readable, and the inline comment clearly explains the rationale behind the change, making this a thoughtful and well-documented improvement.