`Re.split_full` returns `[]` when passed `""`

[dra27]

Both Re.split and Re.split_full should return [`Text ""] and [""] respectively for any regular expression when passed "".

[vouillon]

I think it makes a lot of sense that a `Text "" is included in the output of Re.split_full only between delimiters.

#233 adds a Re.split_delim function which returns [""] when passed the empty string (and a regular expression which does not match the empty string).