json: be more careful when iterating through a JSON object/array

Let's exit the loop early in case the variant is not actually an object
or array. This is safer since otherwise we might end up iterating
through these variants and access fields that aren't of the type we
expect them to be and then bad things happen.

Of course, this doesn't absolve uses of these macros to check the type
of the variant explicitly beforehand, but it makes it less bad if they
forget to do so.