Change get_slot_value function to return None when slot is not present

[Shreyas-vgr]

Change get_slot_value function to return None when slot is not present

Description

Instead of throwing an error, it would be better to return null when the slot is not present in requestEnvelope. Thus developer don't need to check whether the slot is existed before calling this util function.

Motivation and Context

See above description

Testing

Changed the unit test and all unit tests passed.

Screenshots (if appropriate)

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)

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

License

• I confirm that this pull request can be released under the Apache 2 license