converted size expr evaluate to 0 on NULL field

[vmora]

fix #12888
use coalesce(expr,0) to have zero size if expression evaluates to NULL
otherwise the default size would be used

[vmora]

@nyalldawson As I mentioned in the ML "[Qgis-developer] Bug in symbol size data-defined override"

The default size when the expression evaluate to NULL will also be an issue with the size assistant. Maybe we should use coalesce( expr, 0 ) in QgsScaleExpression.

[nyalldawson]

@vmora sorry, forgot to merge this. But I'm interested in other opinions regarding the null behaviour. I think for everything non-size related then we definitely should fallback from null to the user set value, but I'm not 100% sure about sizes. What's the use case for having data defined size with null values as hidden symbols anyway?

[vmora]

@nyalldawson I made a PR to have the assistant generate size expr with the coalesce #2128

IMO, when your symbol size is DD, then the size carries information, if you don't have the information, you don't want it to have a misleading default value. This was the old behavior and I believe it should be maintained.

Let's see the answer on the ML before merging #2128

[haubourg]

@nyalldawson Hi Nyall, from a user point of view, this is really strange to have a NULL fall back to a value not set inside the expression. I expect, and QGIS 2.6 (or 2.4, can't remember) did that, to always see points, and NULL values should only show minimum possible size.
This avoids masking some objects, and avoids caveats in some use cases such as selection by hand, not showing all.
Cheers,
Régis

[nyalldawson]

@vmora should i merge this? what was the final decision?

[vmora]

@nyalldawson yes, this one maintains the old behaviour, so I think this should be merged.

@haubourg #2128 now gives control to the user for the size when expression is NULL.

[nyalldawson]

merged in 4fb22b4, thanks!