scriblib/autobib: more graceful failure when bib not rendered

original commit: 6a5f73d3d9e4e9570c657f77ca8bf20c3510a2b8

collects/scriblib/autobib.rkt

@@ -69,6 +69,7 @@
                                                         (define maybe-disambiguation
                                                           (resolve-get part ri `(autobib-disambiguation ,key)))
                                                         (case maybe-disambiguation
+                                                          [(#f) #f]
                                                           [(unambiguous) #f]
                                                           [else (make-link-element #f maybe-disambiguation `(autobib ,key))]))
                                                       ","))

collects/tests/scribble/docs/autobib-broken.scrbl

@@ -0,0 +1,17 @@
+#lang scribble/base
+@(require scriblib/autobib)
+
+@(define-cite cite citet gen)
+
+@(define redex
+  (make-bib
+    #:author (authors "Matthias Felleisen" "Robert Bruce Findler" "Matthew Flatt")
+    #:title "Semantics Engineering with PLT Redex"
+    #:location (book-location #:publisher "MIT Press")
+    #:is-book? #t
+    #:date "2010"))
+
+Since there's no bibliography, this link won't work:
+@cite[redex].
+
+

collects/tests/scribble/docs/autobib-broken.txt

@@ -0,0 +1 @@
+Since there’s no bibliography, this link won’t work:  (??? ???).