string_array_add pipeline function: Add handling for null elements argument

[danotorrey]

Description

Add handling for condition when the elements string_array_add pipeline function receives a null value, such as when a non-existent field (e.g.$message.field_does_not_exist) is passed to it.

Now, the function correctly returns the supplied value

Motivation and Context

Fixes https://github.com/Graylog2/graylog-plugin-enterprise/issues/7262

How Has This Been Tested?

To manually test this create a pipeline rule with the following rule (supplying the function a null value), and use the Rule simulator below, to ensure that the correct [ "test1", "test2" ] value is returned, and that the error mentioned in the issue does not occur.

rule "test"
when
    true
then
    let t = string_array_add (
        elements: $message.null_field,
        value: [ "test1", "test2" ]
        );
        
    set_field ( field: "_test", value: t );
end

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (non-breaking change)
• [ ] 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 CONTRIBUTING document.
• [ ] I have added tests to cover my changes.