Add noNullProps option

[jakelazaroff]

Since TypeScript can't typecheck tagged template strings, I've been compiling my code using babel-plugin-htm and running TypeScript on the output. That works pretty well, with one big exception: omitting the props entirely passes null rather than an empty object:

function Foo(props: { required: string }) {
  // ...
}

function Bar() {
  return html`<${Foo} />`;
  // return h(Foo, null);
}

h's type signature allows null to be used in place of any props object, which means the preceding code works fine:

export function h<P>(
	type: ComponentType<P> | string,
	props: (Attributes & P) | null,
	...children: ComponentChildren[]
): VNode<Attributes & P>;

This PR introduces a noNullProps option to the Babel plugin, which if set to true passes an empty object instead of null when there are no props.