Fix return type of intent request util functions

[arnaudcourtecuisse]

Description

These function can return nullish values but are declared to return a non-nullish type (either Slot or SlotValue or string)
I fixed the return type declaration to match the implementation.

Motivation and Context

It fixes issue #701

Testing

I used npm link so that my skill uses my local version of ask-sdk-core. Then, with ngrok, I exposed my local skill endpoint and tested it with my Alexa device.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Docs(Add new document content)
• Translate Docs(Translate document content)

Breaks TS builds that handled the nullish value at runtime with a try/catch.

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed
• My commit message follows Conventional Commit Guideline

License

• By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.