fix(types): encode nil slices as empty arrays in Allow type

[Kubudak90]

Fixes #62

Problem

By default, Go's json package marshals nil slices as null, whereas empty slices are marshaled as []. This causes issues for clients in other languages (e.g., JavaScript) that may choke on null when expecting an array.

Solution

Added a custom MarshalJSON method to the Allow type that converts nil slices to empty slices before marshaling. This ensures consistent encoding as [] regardless of whether the slice is nil or empty.

Changes

• Added MarshalJSON method to Allow type
• Nil slices for OperationStatuses, OperationTypes, Errors, CallMethods, and BalanceExemptions are now encoded as [] instead of null
• Added test file nil_slice_test.go with tests for nil and empty slice marshaling

Testing

The fix ensures:

• nil slices → [] in JSON
• Empty slices → [] in JSON (unchanged behavior)
• Non-empty slices → original values in JSON (unchanged behavior)

This is a backward-compatible change that only affects the JSON representation of nil slices, making the API more robust for cross-language compatibility.

[cb-heimdall]

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