types: Fix Arity function to return zero when type is known (#3932)
types: Fix Arity function to return zero when type is known (#3932)

This commit updates the Arity function to return zero if the type is
known and -1 if the type is unknown. This makes the Arity function
behave the same as the GetArity function on the compiler which was
used for similar purposes.

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>
open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa