[processing][needs-docs] allow to exclude features without category from GRASS export

[alexbruy]

Description

Ressurects #8419. IMHO this is quite important fix and would be nice to have in both 3.6 and 3.4 LTR.

As per #8419 while exporting GRASS vectors to a simple features format using v.out.ogr now the -c flag is always used, and this is not expected as it can lead to wrong/unexpected results. On other hand, to keep API stable and produce same results as older 3.x version this PR keeps -c flag enabled by default, allowing user to change it.

If we argee that better to disable -c flag by default, I will update all tests accordingly.

@gioman hope you are fine with it.

Checklist

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[gioman]

@alexbruy totally fine!

[alexbruy]

@nyalldawson, @gioman if you are ok with these changes I will merge it and backport to 3.4 today

[nyalldawson]

No strong opinions here - if @gioman is happy, I'm happy!

[gioman]

No strong opinions here - if @gioman is happy, I'm happy!

yes, I think that the flag was added without fully understanding the consequences.

[alexbruy]

@gioman so using it by default is ok for you (this way algorithms behavior will be the same as in previous versions) or better to turn it off by default?

[gioman]

@alexbruy the "-c" flag must not be used by default when exporting vectors from GRASS/Processing. It should eventually be possible to enable it (this is what my PR did, and I think also yours). In QGIS 2 this flag was not used (and had no option to enable it, as far as I remember).

[alexbruy]

@gioman I know. But in previous QGIS 3.x releases it was used and probably some users now rely on this wrong behavior. I'm not sure if we should break it now. Personally I'm fine with disabling -c flag by default and updating all related test.

[gioman]

But in previous QGIS 3.x releases it was used

to me it was added by mistake, not thinking about the consequences

probably some users now rely on this wrong behavior

I doubt, in my opinion this option leads to a wrong output most of the times.