routing: don't set custom amount if manager isn't handling

routing/bandwidth.go

@@ -143,6 +143,14 @@ func (b *bandwidthManager) getBandwidth(cid lnwire.ShortChannelID,
 					"auxiliary bandwidth: %w", err))
 			}
 
+			// If AuxBandwidth returns None, it means the external
+			// traffic shaper is not handling the channel. In that
+			// case, we'll just return the original bandwidth and
+			// no custom amount.
+			if auxBandwidth.IsNone() {
+				return fn.Ok(bandwidthResult{})
+			}
+
 			// We don't know the actual HTLC amount that will be
 			// sent using the custom channel. But we'll still want
 			// to make sure we can add another HTLC, using the