Fix errors in default() function

[johakoch]

use unsafe unified return type (like in stdlib.CoalesceFunc) from arguments ignoring cty.NilType arguments; and returning cty.NilType if all arguments are cty.NilType arguments (as expected by testcase default(request.cookies.undef, request.cookies.undef))

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Reviewer checklist

• Read PR description: a summary about the changes is required
• Changelog updated
• Documentation: docs/{Reference, Cli, ...}, Docker and cli help/usage
• Pulled branch, manually tested
• Verified requirements are met
• Reviewed the code
• Reviewed the related tests