Given an object or array obj, return a compact object.
A compact object is the same as the original object, except with keys containing falsy values removed. This operation applies to the object and any nested objects. Arrays are considered objects where the indices are keys.
A value is considered falsy when Boolean(value) returns false.
You may assume the obj is the output of JSON.parse. In other words, it is valid JSON.
To compact an object or array, recursively remove all keys (or indices) with falsy values. For arrays, filter out falsy elements and recursively compact nested arrays/objects. For objects, remove keys with falsy values and recursively compact nested values.