HTML writer: Don't create invalid data- attribute...

for empty attribute key. (It would be better to make these
unrepresentable in the type system, but for now this is
an improvement.)

Closes #7546.

src/Text/Pandoc/Writers/HTML.hs

@@ -633,6 +633,7 @@ toAttrs kvs = do
          return (keys, attrs)
        else return (Set.insert k keys, addAttr html5 mbEpubVersion k v attrs)
   addAttr html5 mbEpubVersion x y
+    | T.null x = id  -- see #7546
     | html5
       = if x `Set.member` (html5Attributes <> rdfaAttributes)
              || T.any (== ':') x -- e.g. epub: namespace

test/command/7546.md

@@ -0,0 +1,6 @@
+```
+% pandoc -t html -f native
+Span ("", [], [("","")]) []
+^D
+<span></span>
+```