Fix crash when parsing function with 'return;'

lib/jsduck/function_ast.rb

@@ -37,7 +37,7 @@ module JsDuck
     end
 
     def return_this?(ast)
-      return?(ast) && this?(ast["argument"])
+      return?(ast) && !!ast["argument"] && this?(ast["argument"])
     end
 
     def return?(ast)

spec/function_ast_spec.rb

@@ -19,6 +19,10 @@ describe "JsDuck::FunctionAst#chainable?" do
     chainable?("/** */ function foo() {}").should == false
   end
 
+  it "false when body has empty return statement" do
+    chainable?("/** */ function foo() { return; }").should == false
+  end
+
   it "true when single RETURN THIS statement in body" do
     chainable?("/** */ function foo() {return this;}").should == true
   end