types: expand selectedItem to include null

[mattrothenberg]

What:

Expands the type definition of selectedItem in both the useSelect and useCombobox interfaces so that TS developers can effectively control their downshift instance without having to suppress warnings and compilation errors.

Note that this change isn't necessary (at least I don't think) for useMultipleSelection since passing an empty array effectively controls the Downshift.

Why:
#1088

As per the docs, selectedItem can be either:

• A value, resulting in a controlled Downshift
• Null, resulting in a controlled Downshift
• Undefined, resulting in an uncontrolled Downshift

Up to this point, that second state wasn't possible to achieve in TS land due to type errors.

How:

Expanded the type definition to include null.

Checklist:
I'm not sure which of these are applicable 🤔

• Documentation
• Tests
• TypeScript Types
• Flow Types
• Ready to be merged