REST API cannot handle {} values (empty associative arrays)

Suppose civicrm_api() returns any value with array().

For example: civicrm_api3('Mailing','get',...) might return a $result where template_options is array().

Now suppose we call this via REST. This runs civicrm_api and passes the output to json_encode for encoding. Since PHP treats numeric arrays and associative arrays as the same thing, it is ambiguous whether json_encode should output that as "[]" (empty JSON numeric array) or as "{}" (empty JSON associative array).

This leads to subtle buggy symptoms. Continuing our example where "template_options" is misconstrued as "[]" instead of "{}", the client will happily parse this, and JS code can even work with it. The statement mailing.template_options.foo = "bar"; would execute because "template_options" is an object of type Array. However, it cannot be serialized back to JSON correctly – the "foo" property will be ignored (because JS Array's are supposed be converted to JSON "[]").

I'm really not sure how to fix this generally – one way or another, it feels like we'd need to have special-case logic or metadata (either on the PHP-side for encoding or the JS-side for decoding) which says "field #1 is intended to be numeric-array, and field #2 is intended to be associative-array".