QQuickStyledText: do not insert an extra blank line in the beginning

Do not insert an extra line-separator when the document begins with
an element such as <p> or <h1> that causes an implicit line break.

Task-number: QTBUG-38531
Change-Id: Ib1da9df9a4a767cf11c63a4e2845f334cd69df69
Reviewed-by: Alan Alpert <aalpert@blackberry.com>