Docx writer: allow empty paragraphs.

See #2252.

This also changes fixDisplayMath from Text.Pandoc.Writers.Shared
so that it no longer produces empty Para as an artifact.
(That was the original reason the writer omitted them.)

src/Text/Pandoc/Writers/Docx.hs

@@ -922,8 +922,6 @@ blockToOpenXML' opts (Para [Image attr alt (src,'f':'i':'g':':':tit)]) = do
   captionNode <- withParaProp (pCustomStyle "ImageCaption")
                  $ blockToOpenXML opts (Para alt)
   return $ mknode "w:p" [] (paraProps ++ contents) : captionNode
--- fixDisplayMath sometimes produces a Para [] as artifact
-blockToOpenXML' _ (Para []) = return []
 blockToOpenXML' opts (Para lst) = do
   isFirstPara <- gets stFirstPara
   paraProps <- getParaProps $ case lst of

src/Text/Pandoc/Writers/Shared.hs

@@ -196,13 +196,19 @@ fixDisplayMath :: Block -> Block
 fixDisplayMath (Plain lst)
   | any isDisplayMath lst && not (all isDisplayMath lst) =
     -- chop into several paragraphs so each displaymath is its own
-    Div ("",["math"],[]) $ map (Plain . stripLeadingTrailingSpace) $
+    Div ("",["math"],[]) $
+       map Plain $
+       filter (not . null) $
+       map stripLeadingTrailingSpace $
        groupBy (\x y -> (isDisplayMath x && isDisplayMath y) ||
                          not (isDisplayMath x || isDisplayMath y)) lst
 fixDisplayMath (Para lst)
   | any isDisplayMath lst && not (all isDisplayMath lst) =
     -- chop into several paragraphs so each displaymath is its own
-    Div ("",["math"],[]) $ map (Para . stripLeadingTrailingSpace) $
+    Div ("",["math"],[]) $
+       map Para $
+       filter (not . null) $
+       map stripLeadingTrailingSpace $
        groupBy (\x y -> (isDisplayMath x && isDisplayMath y) ||
                          not (isDisplayMath x || isDisplayMath y)) lst
 fixDisplayMath x = x