Improved TypeScript definition

• Fixed TypeScript definition for getItem. It now notes that getItem can return null, so this change may cause TypeScript code that didn't account for null values to fail. See #980.

This was shipped as a minor release as it has the potential to break TypeScript checks that weren't checking for a null return value from getItem. Note that this version otherwise introduces no new features, so you can upgrade later when you're ready to fix code to check for null return values for getItem calls.