Dialogs: fix warning in DefaultWindowDecoration

content can be null

Change-Id: I69e63fa9f9652103dab7b295c06d56c18ee78933
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
DefaultMessageDialog: ensure contentColumn initial width

anchors seem to be applied after the Flow does its initial layout,
but the Flow needs to know how wide it will be.  The direct width
binding is easier to calculate and applied earlier.

Task-number: QTBUG-41734
Change-Id: I6232ef5ec82396f271a38f1068bc453872fedb35
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>