Fix an infinite recursion issue when registering a Symbol type with a `nil` packer

[casperisfine]

That would cause has_symbol_ext_type = true to be set, so msgpack_packer_write_symbol_value would invoke Symbol#to_msgpack which itself would call back to write_symbol causing infinite recursion.

cc @peterzhu2118 @soyed

[casperisfine]

Urk, of course the Java side need fixing too. I'll deal with it tomorrow.