Fix bug where whitespace in an empty node was doubled

<table>\n\n</table> round-tripped to <table>\n\n\n\n</table> because
we would store '\n\n' in both the innerPre and innerPost fields.
Fixed by not setting innerPost if the element is empty.

Change-Id: I0393bfaf9793fdebc8fff72c8760113fa69bb2bd
Merge "Fix bug where whitespace in an empty node was doubled"