only search for classes if a stylesheet is provided

[martinklepsch]

fixes #159

While in #159 the root cause was described as a missing class I think it's actually a nil stylesheet that will cause the NPE when (map stylesheet class) is called.

All of the following invocations should now work fine:

(pdf
 [{:stylesheet {:custom {:size 40}}}
  [:phrase.custom "Styled"]]
 "test.pdf")

;; worked
(pdf
 [{:stylesheet {:wrong {:size 10}}}
  [:phrase.custom "Styled"]]
 "test.pdf")

;; failed, works now
(pdf
 [{}
  [:phrase.custom "Styled"]]
 "test.pdf")

;; failed, works now
(pdf
 [nil
  [:phrase.custom "Styled"]]
 "test.pdf")

;; failed, works now
(pdf
 [{:stylesheet nil}
  [:phrase.custom "Styled"]]
 "test.pdf")

About the code:

• The fallback could also be handled in get-class-attributes (?)
• I'm not sure what the cond-> in line 39 is good for since merging nil is the same as not merging anything.

[yogthos]

Thanks, I just pushed out 2.3.1 with the fix.