ToParentBlockJoinSortField does not handle missing values correctly #15548
Description
Description
I realised that as part of #15483 I'd made it impossible to set a missing value on ToParentBlockJoinSortField, so set about adding the functionality back in and adding some tests. This showed pretty quickly that the missing value functionality here doesn't really work in the way I'd expect.
The idea of this sort field is that you query parent documents, but order them by the 'best' value of their children. So a missing value should by rights be applied to a child document that doesn't have a value, as it's the children's values that are being used for ordering. However, what actually happens is that the missing value is used for parent documents that have either no children at all, or only children without sort values.
I think we should change this so that the missing value is propagated to the child sorters.
Version and environment details
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