Change QLineF::setLength() to work whenever length() is non-zero

Previously it only worked when isNull() was false, which is true for very short lines, even though length() may be non-zero.

[ChangeLog][QtCore][QLineF] QLineF::setLength() will now set the length if the line's length() is non-zero. Previously, it was documented to only set the length if isNull() was false; this is a fuzzy check, so isNull() could be true for a line with non-zero length().

Fixes: QTBUG-89569 Pick-to: 6.0 5.15 Change-Id: I803e622ad09c85815dde25df8dd3ba6dfcba0714 Reviewed-by: Lars Knoll lars.knoll@qt.io (cherry picked from commit 69747376)