2D: Fix QLogValueAxis accepting invalid values

QLogValueAxis setters accepted invalid values which would eventually
cause crash. This patch changes it to reject and warn about them. In
addition, the auto tests needed to be adjusted.

[ChangeLog][Graphs2D] QLogValueAxis::setMin rejects 0.0 but the default
value was 0.0. Now the default is 1.

Pick-to: 6.12
Fixes: QTBUG-150973
Change-Id: I18a1e1b48ecc407fdf2862d763b954e198db8a6f
Reviewed-by: Tomi Korpipaa <tomi.korpipaa@qt.io>