fix: don't try and fix producer keys for a nil string

Not sure how there is a nil string here, but at least it will more the error somewhere less confusing.

Author: bethesque

lib/pact/consumer_contract/consumer_contract.rb

@@ -59,7 +59,7 @@ def self.from_uri uri, options = {}
     end
 
     def self.maintain_backwards_compatiblity_with_producer_keys string
-      string.gsub('"producer":', '"provider":').gsub('"producer_state":', '"provider_state":')
+      string.gsub('"producer":', '"provider":').gsub('"producer_state":', '"provider_state":') if string
     end
 
     def find_interaction criteria