Harden string functions when NULL is passed

[colincadams]

Description of the change

Many of the existing string function implementations fail when they receive NULL with an expected STRING or BYTES error. This adds a bunch of tests with what happens when NULL is passed to each param of these string functions in BQ (returns NULL or empty array) and updates the implementation to match.

Type of change

All pull requests must have at least one of the following labels applied (otherwise the PR will fail):

Label	Description
Type: Bug	non-breaking change that fixes an issue
Type: Feature	non-breaking change that adds functionality
Type: Breaking Change	fix or feature that would cause existing functionality to not work as expected
Type: Non-breaking refactor	change addresses some tech debt item or prepares for a later change, but does not change functionality
Type: Configuration Change	adjusts configuration to achieve some end related to functionality, development, performance, or security
Type: Dependency Upgrade	upgrades a project dependency - these changes are not included in release notes

Related issues

Root cause of #20740, and many of the todos tagging #20752 which I incorrectly labeled.

Checklists

Development

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These boxes should be checked by the submitter prior to merging:

• Tests have been written to cover the code changed/added as part of this pull request

Code review

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[ageiduschek]

Nice