fix(radio-group): support compareWith in deselection, tabindex and handle null values safely

[rudrakshagarwal4200-ai]

Description

When using compareWith with property name or custom function on ion-radio-group:

1. Passing null as the current value threw a TypeError (Cannot read properties of null) in compareOptions when accessing currentValue[compareWith].
2. In radio-group.tsx, onClick and onKeydown checked newValue !== currentValue using strict reference equality, preventing allowEmptySelection from deselecting an option when values matched by compareWith but were distinct object instances.
3. setRadioTabindex used strict reference equality (radio.value === value) instead of isOptionSelected(value, radio.value, this.compareWith), causing incorrect roving tabindex focus for object values.

This PR fixes these issues by:

1. Making property-access safe against null / undefined values in compareOptions and returning false early in isOptionSelected when currentValue is null or undefined.
2. Using isOptionSelected in onClick and onKeydown to properly support allowEmptySelection deselection with compareWith.
3. Using isOptionSelected in setRadioTabindex to find the checked radio button.
4. Adding unit tests in core/src/components/radio-group/test/radio-group.spec.tsx.

Fixes #30560

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