Some minor improvements have been made to built-in utility functions. (GL1710)
- A new `expression_text()` beta function has been added to assist expert users
with pipeline troubleshooting
- New `assert_present()` and `is_nothing()` beta functions have been added to
help detect if pipeline data has the special 'nothing' type.
- `is_not_null()` and `str()` now produce an error if given no arguments


