Even though the Javadoc for Constants#toCode and #toCodeForSuffix
specifies that a null value for the 'namePrefix' and 'nameSuffix'
parameters are respectively allowed, before this change passing a null
to either would result in a NullPointerException.
This change fixes constant name lookup for null values of these params
as if an empty string had been passed instead. This way of handling a
null value is consistent with the rest of Constants class API.
Issue: SPR-8278