Powerpoint writer: avoid extra blank lines before author.

(In the case where there is no subtitle.)

Closes #10619.

src/Text/Pandoc/Writers/Powerpoint/Output.hs

@@ -1752,7 +1752,9 @@ metadataToElement layout titleElems subtitleElems authorsElems dateElems
   , Just cSld <- findChild (elemName ns "p" "cSld") layout
   , Just spTree <- findChild (elemName ns "p" "spTree") cSld = do
       let combinedAuthorElems = intercalate [Break] authorsElems
-          subtitleAndAuthorElems = intercalate [Break, Break] [subtitleElems, combinedAuthorElems]
+          subtitleAndAuthorElems = intercalate [Break, Break] $
+                                    filter (not . null)
+                                     [subtitleElems, combinedAuthorElems]
       (titleId, titleElement) <- nonBodyTextToElement layout [PHType "ctrTitle"] titleElems
       (subtitleId, subtitleElement) <- nonBodyTextToElement layout [PHType "subTitle"] subtitleAndAuthorElems
       (dateId, dateElement) <- nonBodyTextToElement layout [PHType "dt"] dateElems