Make 'mapconcat' argument 'separator' optional

* src/fns.c (Fmapconcat): Make third 'separator' argument
optional.  (Bug#50965)
* doc/lispref/functions.texi (Mapping Functions): Update
documentation for above change.
* test/src/fns-tests.el (fns-tests-mapconcat): New test.

* doc/misc/cl.texi (Obsolete Setf Customization): Don't use third
mapconcat argument in example.
* lisp/emacs-lisp/subr-x.el (string-join): Doc fix.

doc/lispref/functions.texi

@@ -961,14 +961,14 @@ side-effects only---the values it returns are ignored, not collected
 into a list.  @code{mapc} always returns @var{sequence}.
 @end defun
 
-@defun mapconcat function sequence separator
+@defun mapconcat function sequence &optional separator
 @code{mapconcat} applies @var{function} to each element of
 @var{sequence}; the results, which must be sequences of characters
 (strings, vectors, or lists), are concatenated into a single string
 return value.  Between each pair of result sequences, @code{mapconcat}
 inserts the characters from @var{separator}, which also must be a
-string, or a vector or list of characters.  @xref{Sequences Arrays
-Vectors}.
+string, or a vector or list of characters; a @code{nil} value is
+treated as the empty string.  @xref{Sequences Arrays Vectors}.
 
 The argument @var{function} must be a function that can take one
 argument and returns a sequence of characters: a string, a vector, or
@@ -986,8 +986,7 @@ string.
 
 @group
 (mapconcat (lambda (x) (format "%c" (1+ x)))
-           "HAL-8000"
-           "")
+           "HAL-8000")
      @result{} "IBM.9111"
 @end group
 @end example

doc/misc/cl.texi

@@ -5030,7 +5030,7 @@ The above @code{incf} example could be written using
 @ignore
 (defmacro concatf (place &rest args)
   (gv-letplace (getter setter) place
-    (macroexp-let2 nil v (mapconcat 'identity args "")
+    (macroexp-let2 nil v (mapconcat 'identity args)
       (funcall setter `(concat ,getter ,v)))))
 @end ignore
 @end defmac