plot_tag_status_by_subquadrat() should map empty pages

[dkenfack]

For a recensus, It will be useful to produce all maps of a column (100 maps) including those that did not have trees in the previous census.

[maurolepore]

Thanks @dkenfack!

This is a great suggestion. I'll explain to myself the issue so I can work on it later.

I understand that -- right now -- the function maply_tag() outputs no map if an entire page has no tree in it. That is a problem because trees may have recruited in that previously empty page_ and fieldworkers would have nowhere to map them.

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