fix(select-widget): select widget not able to select number value 0

[imangd]

Fixes #6515

Summary
When a select widget which has a value of 0 is selected, if isMultiple is set to false, null is selected and if isMultiple is set to true, an error is thrown. This happens because in optionToString function in SelectControl, the conditional checks for truthy value which excludes numerical 0 and returns null if a value of 0 is selected.

This PR proposes changes to this logic by explicitly checking the value type to be either number or string.
It also includes changes to the propTypes of SelectControl to match the types in schema.js.

Test plan

1. Under dev-test/config.yml, replace options values ['a', 'b', 'c'] with [{ label: 'a', value: 0 }, { label: 'b', value: 1 }, { label: 'c', value: 2 }]
2. Run dev-test locally: npm run start
3. Go to Kitchen Sink and select option a for both select widgets

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