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.