chore: fix node break issue when RLN is unregistered (#3573)

Author: darshankabariya

waku/waku_rln_relay/group_manager/on_chain/group_manager.nim

@@ -202,11 +202,15 @@ proc trackRootChanges*(g: OnchainGroupManager) {.async: (raises: [CatchableError
       let rootUpdated = await g.updateRoots()
 
       if rootUpdated:
-        let proofResult = await g.fetchMerkleProofElements()
-        if proofResult.isErr():
-          error "Failed to fetch Merkle proof", error = proofResult.error
-        else:
-          g.merkleProofCache = proofResult.get()
+        ## The membership set on-chain has changed (some new members have joined or some members have left)
+        if g.membershipIndex.isSome():
+          ## A membership index exists only if the node has registered with RLN.
+          ## Non-registered nodes cannot have Merkle proof elements.
+          let proofResult = await g.fetchMerkleProofElements()
+          if proofResult.isErr():
+            error "Failed to fetch Merkle proof", error = proofResult.error
+          else:
+            g.merkleProofCache = proofResult.get()
 
         let nextFreeIndex = await g.fetchNextFreeIndex()
         if nextFreeIndex.isErr():