[finagle-mysql] Give NullParameter a friendly string representation

Otherwise stringifying it results in random output like:
 `com.twitter.finagle.mysql.Parameter$NullParameter$@5550f534`.

This doesn't work well with e.g. snapshot tests.

Result

Closes #947

Differential Revision: https://phabricator.twitter.biz/D1107769

finagle-mysql/src/main/scala/com/twitter/finagle/mysql/Parameter.scala

@@ -179,6 +179,7 @@ object Parameter {
     type A = Null
     def value: Null = null
     def evidence: CanBeParameter[Null] = CanBeParameter.nullCanBeParameter
+    override def toString: String = "Parameter(null)"
   }
 }