autopilot: fix nil map assignment

Use the `clear` call to reset a map on `reset` instead of assigning nil
to the entry.

discovery/bootstrapper.go

@@ -249,7 +249,7 @@ func (c *ChannelGraphBootstrapper) SampleNodeAddrs(_ context.Context,
 
 			return errFound
 		}, func() {
-			a = nil
+			clear(a)
 		})
 		if err != nil && !errors.Is(err, errFound) {
 			return nil, err