Return empty array if there is no aspects for a category in get_item_aspects_for_category endpoint

[ulandj]

Problem

The get_item_aspects_for_category endpoint returns inconsistent data types:

• Returns true when eBay responds with HTTP 204 (no aspects available)
• Returns { "aspects" => [...] } when aspects exist

This inconsistency forces consumers to handle two different data types.

Solution

Override the default 204 handler to always return { "aspects" => [] } when no aspects are available, ensuring a consistent hash structure regardless of the response status.

Changes

• Added explicit response(204) handler in the operation definition
• Returns empty aspects array for consistency
• Added test coverage for the 204 No Content scenario