message: update method signature descriptor
diff: @@ -4,11 +4,13 @@ import { GetNullDescriptor } from "../null/null";
  import { GetDescriptor } from "../descriptor";
  export function GetMethodSignatureDescriptor(node: ts.MethodSignature): ts.Expression {
  - const hasReturnValue = node.type;
  + let returnType: ts.Expression;
  - if (hasReturnValue) {
  - return GetMethodDescriptor(node.name, GetDescriptor(node.type));
  + if (node.type) {
  + returnType = GetDescriptor(node.type);
  + } else {
  + returnType = GetNullDescriptor();
  }
  - return GetMethodDescriptor(node.name, GetNullDescriptor());
  + return GetMethodDescriptor(node.name, returnType);
  }
  