Update form.ts to handle null discription

[Darshan808]

Fixes #33

Issue: When there is no description in Google Forms, the value is set to null. Since the current code only handles description as undefined or a string, TypeScript raises a type incompatibility error.

Type 'null' is not assignable to type 'string | undefined'.

Solution: Added optional null type to the description field to handle this error.

[cnkeats]

Also encountered this, though it was easily mitigated with a description. Still a good change.