Fix for issuing element key:value combination breaks Tobago

This is a follow up issue caused by the fix for https://issues.apache.org/jira/browse/MYFACES-4606

raised by the Tobago guys.

The fix broke one of their testcases!

https://github.com/henningn/myfaces-tobago.git

4 checkboxes on the page, example does not work anymore because

• git checkout jsfjs_next_gen402 
• mvn clean install
• cd tobago-example/tobago-example-demo 
• mvn clean -Pdev -Pjetty jetty:run
• open http://localhost:8080/content/030-select/50-selectManyCheckbox/Multi_Checkbox.xhtml

The request adds the issuing element with a null value!