fix: use empty map instead of nil value for setting BaseApp.snapshotManager.extensions (#12031)

## Description

`BaseApp` method `SetSnapshot` sets `BaseApp.snapshotManager.extensions` with a nil value, this will cause nil map error whenever `BaseApp.snapshotManager.extensions` is used, so I change it to an empty map instead.

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snapshots/manager.go

@@ -70,6 +70,9 @@ var ErrOptsZeroSnapshotInterval = errors.New("snaphot-interval must not be 0")
 
 // NewManager creates a new manager.
 func NewManager(store *Store, opts types.SnapshotOptions, multistore types.Snapshotter, extensions map[string]types.ExtensionSnapshotter, logger log.Logger) *Manager {
+	if extensions == nil {
+		extensions = map[string]types.ExtensionSnapshotter{}
+	}
 	return &Manager{
 		store:      store,
 		opts:       opts,