Fix cross_tab column naming edge cases, add fill_empty

[radeusgd]

Pull Request Description

Closes #5151 and adds some additional tests for cross_tab that verify duplicated and invalid names.

I decided that for empty or Nothing names, instead of replacing them with Column and implicitly losing connection with the value that was in the column, we should just error on such values.

To make handling of these easier, fill_empty was added allowing to easily replace the empty values with something else.

Also, {is,fill}_missing was renamed to {is,fill}_nothing to align with Filter_Condition.Is_Nothing.

Important Notes

Checklist

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