pdf-annot: Fix: Render color only if it's defined.

Free-text annotations do not have a color associated with them, which
causes annotation listing to crash (since the default value of
`pdf-annot-list-highlight-type` is now true). This bug was introduced
in bb0b71f

lisp/pdf-annot.el

@@ -1652,7 +1652,7 @@ pretty-printed output."
         (type
          (let ((color (pdf-annot-get a 'color))
                (type (pdf-annot-print-property a 'type)))
-           (if pdf-annot-list-highlight-type
+           (if (and pdf-annot-list-highlight-type color)
                (propertize
                 type 'face
                 `(:background ,color