Support for null rounds in `trace_filter` responses

[rjan90]

Tracking issue for following up on the conversation in: https://filecoinproject.slack.com/archives/C096DTH73T6/p1769619594423839

Summary

Goldsky subgraphs fail to index when Filecoin RPC nodes return null instead of [] for trace_filter calls on epochs with no traces (null rounds).

Context

Filecoin has null rounds - epochs that intentionally return nothing. This is normal protocol behavior, but Ethereum tooling (including Goldsky's graph-node) expects an empty array [] rather than null.

Current State

• Call handlers are enabled on mainnet (filecoin) but not testnet (filecoin-testnet)
• Subgraphs break with: Decoder error: Error("invalid type: null, expected a sequence")

Blocker

This blocks settlement tracking on FWSS until Goldsky can handle null responses.

Possible Paths Forward

1. Goldsky updates graph-node to treat null as equivalent to [] for trace responses
2. RPC providers (glif.io, chain.love) change their response format

[rjan90]

It was an issue in Lotus, will be fixed by: filecoin-project/lotus#13483. Will let RPC-providers know once it has been shipped in a Lotus release