fix: prevent nil pointer dereference when admin_nodeInfo omits difficulty

[MysticRyuujin]

Problem

The exporter panics with a nil pointer dereference when calling admin_nodeInfo against Geth (and potentially other clients) that omit the difficulty field from the response. This commonly occurs with post-merge clients where difficulty is no longer relevant.

panic: runtime error: invalid memory address or nil pointer dereference
github.com/ethpandaops/ethereum-metrics-exporter/pkg/exporter/execution/api/types.(*EthProtocol).UnmarshalJSON
    pkg/exporter/execution/api/types/admin.go:53

Root Cause

In EthProtocol.UnmarshalJSON, the code dereferences objMap["difficulty"] without checking if the key exists. When difficulty is missing from the JSON, objMap["difficulty"] is nil and *objMap["difficulty"] causes the panic.

Solution

• Add a nil check before accessing objMap["difficulty"]
• When the key is present: preserve existing behavior (parse as hex string or fall back to standard unmarshal)
• When the key is missing: unmarshal the rest of the struct and leave Difficulty as nil