[Backport queued_ltr_backports] return 0 for count-based attributes in Join attributes by location (summary) Processing algorithm (fix #40108)

[alexbruy]

Description

Manual backport of #66322.

In the Join attributes by location (summary) Processing algorithm non-matching features (features that can not be joined) have NULL for their summaries. While this makes sense for aggregates that depend on field values like min, max, sum, this is wrong for count-based values like count, unique, filled which should use 0.

This happens because for non-matching features the code just resizes the attributes filling all additional attributes with NULLs. Instead we should fill them manually using 0 where this make sense.

Fixes #40108.

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