racket/generic: better error message in empty case

Closes PR 13048

collects/racket/private/generic.rkt

@@ -36,7 +36,7 @@
           (identifier? #'name?)
           (identifier? #'defined-table)
           (let ([generics (syntax->list #'(generic ...))])
-            (and (pair? generics) (andmap identifier? generics))))
+            (and (list? generics) (andmap identifier? generics))))
      (let* ([idxs (for/list ([i (in-naturals 0)]
                              [_ (syntax->list #'(generic ...))])
                     i)]

collects/tests/generic/empty-interface.rkt

@@ -0,0 +1,5 @@
+#lang racket
+(require racket/generic)
+(define-generics name)
+(struct foo ()
+  #:methods gen:name [])

collects/tests/generic/tests.rkt

@@ -11,4 +11,5 @@
          "base-interfaces.rkt"
          "contract.rkt"
          "from-unstable.rkt"
-         "poly-contracts.rkt")
+         "poly-contracts.rkt"
+         "empty-interface.rkt")