Move non-empty-string? to racket/string.

From `unstable/contract`.

pkgs/racket-doc/scribblings/reference/strings.scrbl

@@ -507,6 +507,13 @@ trimmed (which is an alternative to using a @tech{regular expression}
   (string-trim "aaaxaayaa" "aa")
 ]}
 
+@defproc[(non-empty-string? [x any/c]) boolean?]{
+Returns @racket[#t] if @racket[x] is a string and is not empty;
+returns @racket[#f] otherwise.
+@history[#:added "6.2.900.15"]{}
+}
+
+
 @; ----------------------------------------
 @include-section["format.scrbl"]
 

racket/collects/racket/string.rkt

@@ -5,7 +5,8 @@
          string-trim
          string-normalize-spaces
          string-split
-         string-replace)
+         string-replace
+         non-empty-string?)
 
 (define string-append*
   (case-lambda [(strs) (apply string-append strs)] ; optimize common cases
@@ -134,3 +135,6 @@
   (if all?
     (regexp-replace* from* str to*)
     (regexp-replace  from* str to*)))
+
+(define (non-empty-string? x)
+  (and (string? x) (not (zero? (string-length x)))))