Compact protocol map specification bug

[terrelln]

https://www.internalfb.com/intern/staticdocs/thrift/docs/spec/protocol/data/#map-1

It states that:

Maps contain a variable encoded length followed by the key and value types encoded in a single byte. Next comes the payload which is encoded in a similar way to the Binary protocol.

Length	Key Type	Value Type	Key 1	Value 1	...	Key n	Value n
Variable	4-bit	4-bit	Variable	Variable		Variable	Variable

However in the implementation, the byte containing the key type & value type is not present if the length is 0.