In #277, a feature was introduced which made empty string values "" in filter_arg correspond to null when decoded. This change was not reflected in the TypeScript types for the shovel-config-ts config generator; this PR changes the types to allow filter_arg to take an empty string value.

When testing my shovel config with the updated types in this PR, I was successfully able to add "" to a filter_arg without type errors.

Something to consider — in the docs, it says:

filter_arg: Not required when using filter_ref. Use filter_arg when you want to filter on static data. Must be an array of strings. The string values will be decoded depending on the value of comparison. Namely, a string with a 0x prefix will be decoded into bytes, a decimal string will be decoded into uint or int, a string of utf8 characters will be decoded into a string, and an empty string will be decoded as an empty value of the type being compared.

Should the type actually be expanded to (Hex | string)[] here, or even just string[]?