[Select]: size prop defaults to empty string, but its propType does not allow that

[jendowns]

What package(s) are you using?

• carbon-components
• carbon-components-react

Detailed description

In the latest 7.9.0 of carbon-components-react, the size prop is set to an empty string by default:

carbon/packages/react/src/components/Select/Select.js

Line 225 in c9f7e35

size: '',

However, only sm and xl are valid size prop values:

carbon/packages/react/src/components/Select/Select.js

Lines 211 to 215 in c9f7e35

	/**
	* Specify the size of the Text Input. Currently supports either `sm` or `xl` as an option.
	*/
	size: PropTypes.oneOf(['sm', 'xl']),
	};

So the default of the size prop violates the propTypes rules, and it generates errors wherever a "default" size is used. (You can see this is you pull down master / the latest and run the React storybook environment).

[jendowns]

I can help with this 👍 PR coming shortly