Org reader: allow figure with empty caption

A `#+CAPTION` attribute before an image is enough to turn an image into a
figure. This wasn't the case because the `parseFromString` function, which
processes the caption value, would fail on empty values. Adding a newline
character to the caption value fixes this.

Fixes: #3161

src/Text/Pandoc/Readers/Org/Blocks.hs

@@ -288,9 +288,7 @@ blockAttributes = try $ do
   let kvAttrs = foldl' (appendValues "ATTR_HTML") Nothing kv
   let name    = lookup "NAME" kv
   let label   = lookup "LABEL" kv
-  caption' <- maybe (return Nothing)
-                    (fmap Just . parseFromString inlines)
-                    caption
+  caption' <- sequence (parseFromString inlines . (++ "\n") <$> caption)
   kvAttrs' <- parseFromString keyValues . (++ "\n") $ fromMaybe mempty kvAttrs
   return $ BlockAttributes
            { blockAttrName = name

tests/Tests/Readers/Org.hs

@@ -916,6 +916,12 @@ tests =
                     ] =?>
             let attr = ("fig:myfig", mempty, mempty)
             in para (imageWith attr "blub.png" "fig:" "My figure")
+
+        , "Figure with empty caption" =:
+            unlines [ "#+CAPTION:"
+                    , "[[file:guess.jpg]]"
+                    ] =?>
+            para (image "guess.jpg" "fig:" "")
         ]
 
       , "Footnote" =: