ProfileWidget2::plotDive() had this weird interface, where passing
in NULL as dive would mean "show current_dive". However, most callers
would already pass in current_dive. Therefore, unify and always pass
in current_dive if the caller wants to draw the current dive.
This allows us to interpret NULL as "show empty profile". Thus,
passing in current_dive when there is no current_dive simply shows
an empty profile. This makes the calling code in
MainWindow::selectionChanged() simpler.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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