Fix: iOS null exception occurs on Toolkit SfButton when text is null

[KarthikaRSF4617]

Root Cause of the Issue

This issue occurs when the SfButton text is set to null, causing a null exception during iOS text measurement.
The existing implementation directly creates an NSString using:

new NSString(text);

On iOS, NSString does not accept null values, which leads to a null exception at runtime.
This scenario can happen when SfButton or its internal text measurement logic triggers measurement before the text is initialized or when text is explicitly null.

Description of Change

Added a null-coalescing safeguard:

text ??= string.Empty;

Ensures that:

• If text is null, it is replaced with an empty string before creating NSString
• Prevents runtime exceptions from invalid object initialization

This change guarantees:

• Safe text measurement execution on iOS
• No crash when text is missing or not yet initialized
• Improved stability of SfButton rendering lifecycle