Using a 'selectItem' with a 'null' value and a converter that will return 'null' for this value will ignore the value in the html output.
If the value is ignored not renderd in html 'select' tag the submitted value will be the label since no value was provided.
Submitting the form will lead to a conversion error: "None" cannot be converted...
since html takes the label as the value if no value is provided.
Should the desired html not be in which case the conversion would succeed.
Returning empty String would lead to desired behavior. But I would prefer returning null instead, and also because of issue: https://java.net/jira/browse/JAVASERVERFACES_SPEC_PUBLIC-1180