Do not call node.GetChains() if node is nil (#1588)

In `NewSessionNodes`, the variable `node` can be `nil` if the node is
unstaked between `sessionCtx` and `ctx`. In such a case,
`node.GetChains()` causes a panic. This patch makes sure we don't call
it if the `node` is `nil`.
This is a regression from #1582.

## Description

<!-- reviewpad:summarize:start -->
### Summary generated by Reviewpad on 21 Dec 23 10:23 UTC
This pull request fixes a regression introduced in #1582. The patch
ensures that the function `node.GetChains()` is not called if the `node`
is `nil`, preventing a panic in the `NewSessionNodes` function.
<!-- reviewpad:summarize:end -->

x/pocketcore/types/session.go

@@ -150,10 +150,9 @@ func NewSessionNodes(
 
 		// cross check the node from the `new` or `end` world state
 		node = keeper.Validator(ctx, n)
-		lenNodeChains := int64(len(node.GetChains()))
 		// if not found or jailed or is overstaked to chains
 		if node == nil ||
-			(isEnforceMaxChains && lenNodeChains > nodeMaxChains) ||
+			(isEnforceMaxChains && int64(len(node.GetChains())) > nodeMaxChains) ||
 			node.IsJailed() ||
 			!NodeHasChain(chain, node) ||
 			sessionNodes.Contains(node.GetAddress()) {