[ruby/yarp] Remove unnecessary NUL byte in string

https://github.com/ruby/yarp/commit/af867e35b1

yarp/templates/src/node.c.erb

@@ -173,5 +173,5 @@ yp_node_type_to_str(yp_node_type_t node_type)
             return "<%= node.type %>";
 <%- end -%>
     }
-    return "\0";
+    return "";
 }