Markdown writer: preserve figure attributes

0d2114e introduced a bug where Markdown writer would ignore attributes
on the figure if it has class or key-value attributes set.

This commit fixes that and adds a regression test.

Closes #10867

Author: lierdakil

src/Text/Pandoc/Writers/Markdown.hs

@@ -721,7 +721,6 @@ blockToMarkdown' opts (Figure figattr capt body) = do
   let combinedAttr imgattr = case imgattr of
         ("", cls, kv)
           | (figid, [], []) <- figattr -> Just (figid, cls, kv)
-          | otherwise -> Just ("", cls, kv)
         _ -> Nothing
   let combinedAlt alt = case capt of
         Caption Nothing [] -> if null alt

test/command/10867.md

@@ -0,0 +1,26 @@
+```
+% pandoc -f native -t markdown-raw_html
+[ Figure
+    ( "fig:foo" , [] , [ ( "label" , "1.1" ) ] )
+    (Caption
+       Nothing
+       [ Plain
+           [ Str "Figure" , Space , Str "1.1:" , Space , Str "Figure" ]
+       ])
+    [ Plain
+        [ Image
+            ( "" , [] , [] )
+            [ Str "Figure" , Space , Str "1.1:" , Space , Str "Figure" ]
+            ( "./image.png" , "" )
+        ]
+    ]
+]
+^D
+:::: {#fig:foo .figure label="1.1"}
+![Figure 1.1: Figure](./image.png)
+
+::: caption
+Figure 1.1: Figure
+:::
+::::
+```