Add type::is::nil function

[Odonno]

Thank you for submitting this pull request! We really appreciate you spending the time to work on these changes.

What is the motivation?

Add a simple way to check for nullable (not truthy) values.
Note: can be useful for building an ORM.

What does this change do?

Before:

WHERE (value != NONE AND value != null);

After:

WHERE !type::is::nil(value);

What is your testing strategy?

Unit tests

Is this related to any issues?

N/A

Does this change need documentation?

• Add type::is::nil function docs.surrealdb.com#525

Have you read the Contributing Guidelines?

• I have read the Contributing Guidelines

[phughk]

Loving these PRs, thanks!