Allow to specify empty string value in a StaticList with same separator

It is now possible to specify empty string values in a Static list by repeating twice the same separator so for example, with default separators "|", "," and space: ##Foo||Bar## will be considered as the three values "Foo", "" and "Bar", while ##Foo, Bar## will be only considered as the two values "Foo" and "Bar" even if two separators ("," and space) are used.