combobox: Fix onSelectionChange type declaration

[acr92]

onSelectionChange can be invoked with a null key if the user clears the input.

react-spectrum/packages/@react-stately/combobox/src/useComboBoxState.ts

Lines 225 to 229 in c11afb2

	// Set selectedKey to null when the user clears the input.
	// If controlled, this is the application developer's responsibility.
	if (inputValue === '' && (props.inputValue === undefined || props.selectedKey === undefined)) {
	setSelectedKey(null);
	}

This in turns triggers onSelectionChange with a null value, even though the type contract doesn't allow that.

Here's my stacktrace:

SelectUser is my component, which sets defaultItems, selectedKey and onSelectionChange.

✅ Pull Request Checklist:

• N/A Included link to corresponding React Spectrum GitHub Issue.
• N/A - Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• N/A - Filled out test instructions.
• N/A - Updated documentation (if it already exists for this component).
• N/A - Looked at the Accessibility Practices for this feature - Aria Practices

N/A for all as this is just a correction of the type declaration.

[reidbarber]

Thanks for the PR! If you sign the CLA, then close and re-open this PR, it should trigger a re-build that passes CI.

[snowystinger]

Thanks for the PR
Initially I thought this should be moved up to SingleSelection. However, Picker and StepList both extend it as well, and those have selectionChanges which cannot be null.

This falls into our TS Strict #3183 as well