Fix: don't add whitespace in blank lines.

In the example

    script.
      a
        b

        c

Slab was generating whitespaces for the line between b and c (as if
there was a character to be indented as much as b and c). This is no
longer the case.

examples/script.html

@@ -7,3 +7,9 @@
       c
 b
 </script>
+<script>
+    a
+  b
+
+  c
+</script>

examples/script.slab

@@ -5,3 +5,8 @@ script.
 script.
     c
   b
+script.
+  a
+    b
+
+    c

src/Slab/Parse.hs

@@ -106,6 +106,7 @@ parserElement = do
   header <- parserDiv
   parserElemBody ref header
 
+-- | Parse the indented content of an HTML element or a fragment call.
 parserElemBody :: Pos -> ([Block] -> Block) -> Parser (L.IndentOpt Parser Block Block)
 parserElemBody ref header =
   case trailingSym $ header [] of
@@ -154,7 +155,7 @@ textBlock ref p = go
             l <- p
             ls <- go
             let prefix = T.replicate (unPos pos - unPos ref) " "
-                n' = replicate (n - 1) prefix
+                n' = replicate (n - 1) ""
                 l' = prefix <> l
             pure $ n' <> (l' : ls)
 
@@ -164,7 +165,7 @@ textBlock ref p = go
 realign :: [Text] -> [Text]
 realign xs = map (T.drop n) xs
  where
-  n = minimum $ map (T.length . T.takeWhile (== ' ')) xs
+  n = minimum $ map (T.length . T.takeWhile (== ' ')) $ filter (not . T.null) xs
 
 -- | Parse multiple lines starting each with a pipe prefix. Most of our parsers
 -- parse a single line (and optional indented child lines) and most nodes map