[Checkbox] Fix checkbox a11y - screen readers doesn't announce undefined 'checked' (#2699)

* make checkbox a11y annouce null/undefined state

* Change files

* fix small spelling typo

change/@fluentui-react-native-checkbox-ae068286-c9b5-40b3-a0ff-151b9e263620.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "make checkbox a11y annouce null/undefined state",
+  "packageName": "@fluentui-react-native/checkbox",
+  "email": "rohanpd.work@gmail.com",
+  "dependentChangeType": "patch"
+}

packages/components/Checkbox/src/useCheckbox.ts

@@ -107,6 +107,7 @@ export const useCheckbox = (props: CheckboxProps): CheckboxInfo => {
 
 const getAccessibilityState = memoize(getAccessibilityStateWorker);
 function getAccessibilityStateWorker(disabled: boolean, checked: boolean, required: boolean, accessibilityState?: AccessibilityState) {
+  checked = checked ?? false; // Make the value of checked as false when checked is undefined or null for screen reader to announce 'unchecked'
   if (accessibilityState) {
     return { disabled, checked, required, ...accessibilityState };
   }