Clarify `regexp-split' on an empty input.

The text that says that (regexp-split #rx"whatever" "") returns '("")
rather than '() is

  If `input' contains no matches [...] the result is a list containing
  input’s content [...] as a single element.

This is a little implicit, if you consider such an input as having
nothing left to match over so it's as if there is no input (with a port
this confusion is a little clearer).

Clarify with an example in the docs, and also add tests.

collects/scribblings/reference/regexps.scrbl

@@ -785,6 +785,7 @@ an end-of-file if @racket[input] is an input port).
 (regexp-split #rx"" "12  34")
 (regexp-split #rx" *" "12  34")
 (regexp-split #px"\\b" "12, 13 and 14.")
+(regexp-split #rx" +" "")
 ]}
 
 @;------------------------------------------------------------------------

collects/tests/racket/string.rktl

@@ -311,6 +311,9 @@
   (t '("" "" " " "" "" "") eof " *" "12 34")
   (t '(" " "" "" " " "" "" " " "") eof " *" " 12 34 ")
   (t '("" "" " " "" "" "") " " " *" " 12 34 " 1 6)
+  (t '("") eof " *" "")
+  (t '("") eof " *" "1234" 4)
+  (t '("") "34" " *" "1234" 2 2)
   (t regexp-match-positions*)
   (t '((0 . 0) (1 . 1) (2 . 2) (3 . 3)) eof "" "123")
   (t '((0 . 0) (1 . 1) (2 . 3) (3 . 3) (4 . 4) (5 . 5)) eof " *" "12 34")