LaTeX writer: fix line breaks at start of paragraph.

Previously we just omitted these. Now we render them
using `\hfill\break` instead of `\\`.  This is a revision
of a PR by @sabine (#5591) who should be credited with the
idea.

Closes #3324.

test/command/3324.md

@@ -0,0 +1,18 @@
+```
+pandoc -t latex
+Signatures
+
+\
+\
+___________________________\
+Peter Foobar\
+*The Foo Company*
+^D
+Signatures
+
+\hfill\break
+\hfill\break
+\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\\
+Peter Foobar\\
+\emph{The Foo Company}
+```