Don't call splitTreeToNode() with null Node

This patch changes |IndentOutdentCommand::outdentParagraph()| to check whether
|enclosingBlockNode| is null or not, before passing it to
|CompositeEditCommand::splitTreeToNode()| which assumes it isn't null.

This situation can be happened in attach test case, where
|enclosingBlockNode| is calculated from |visibleStartOfParagraph|, which is
second TABLE element, by |enclosingBlock()|.

BUG=378758
TEST=editing/execCommand/outdent-collapse-table-crash.html

Review URL: https://codereview.chromium.org/308023003

git-svn-id: svn://svn.chromium.org/blink/trunk@175251 bbb929c8-8fbe-4397-9dbb-9b2b20218538