WebP: Fix wrong default quality level for writing

If a negative (i.e. illegal/unset) quality value was set to the WebP
handler, it would just bound it to 0 (minimum quality level). This
would happen on every save where no explicit quality level had been
requested on the QImageWriter.
Fix by copying the jpeg handler's behavior: If a negative value is
set, use the default level (75) when storing.

[ChangeLog][WebP handler] Fixed default quality level for writing

Task-number: QTBUG-64437
Change-Id: I0f1cabba6cea6851c6a813bf5bf7ab8e8c49ddfb
Reviewed-by: Andy Shaw <andy.shaw@qt.io>