feat: bypass null subscription filter values

[jensneuse]

Summary

• Add BypassIfValuesNull to subscription field filter config and runtime filter structs.
• Propagate the flag through the planner into resolve filters.
• Treat single-segment templated IN values resolving to nil or GraphQL null as a filter match before event payload lookup.

Test Plan

• cd v2 && go test ./pkg/engine/...

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