Fix crash on #nil in syntaxes

In 3.0.7 (after 0cc7991855 "Ensure
that (syntax ()) results in ("), the use of #nil in syntax-rules
expansions like this:

  (define-syntax foo
    (syntax-rules ()
      ((_ x) (eq? #nil x))))

  (foo #t)

could cause a crash that looks like this:

  ice-9/psyntax.scm:2795:12: In procedure syntax-violation:
  Syntax error:
  unknown location: unexpected syntax in form ()

To fix it, add another special case (the commit mentioned above
special-cased the empty list) to preserve #nil

* module/ice-9/psyntax.scm (gen-syntax): Preserve #nil.
* test-suite/tests/syntax.test: Test #nil in syntax expansions.

Closes: 49305

module/ice-9/psyntax.scm

@@ -2157,6 +2157,7 @@
                       (lambda ()
                         (gen-syntax src #'(e1 e2 ...) r maps ellipsis? mod))
                     (lambda (e maps) (values (gen-vector e) maps))))
+                 (x (eq? (syntax->datum #'x) #nil) (values '(quote #nil) maps))
                  (() (values '(quote ()) maps))
                  (_ (values `(quote ,e) maps))))))
 

test-suite/tests/syntax.test

@@ -1684,6 +1684,16 @@
         (hash interpreted most-positive-fixnum)
       (hash compiled most-positive-fixnum))))
 
+(with-test-prefix "#nil in syntaxes"
+  (pass-if-equal "does not crash"
+      42
+    (let ()
+      (define-syntax foo
+        (syntax-rules ()
+          ;; In 3.0.7 this would crash with
+          ;;   unknown location: unexpected syntax in form ()
+          ((_ x) (when (eq? x #nil) 42))))
+      (foo #nil))))
 
 ;;; Local Variables:
 ;;; eval: (put 'pass-if-syntax-error 'scheme-indent-function 1)