Make sure that syntax-span is not false before checking length

currently rendering scribble with an auto-value (or using #:value with other syntax objects that are numbers) fails.
This fixes the direct problem for me, but I'm not sure it's the best approach.

Example:

#lang scribble/manual

@(module a racket/base
   (provide :a :b)
   (define :a 4.8)
   (define :b 'symbol))

@(require (for-label (submod "." a)))

@defthing[:b symbol? #:auto-value]
@defthing[:a number? #:value 3.5]
@defthing[:a number? #:auto-value] @;; this fails

tested using "Welcome to DrRacket, version 9.1.0.11 [cs]." on windows