dataset: allow nil filter settings

If you do no specify filter settings in the trace2receiver config then
you hit a null pointer exception at runtime as we try to access the
`Keynames` member of a `nil` `*FilterSettings`.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

trace2dataset.go

@@ -522,7 +522,11 @@ func (tr2 *trace2Dataset) exportTraces() {
 		return
 	}
 
-	traces := tr2.ToTraces(dl, tr2.rcvr_base.RcvrConfig.filterSettings.Keynames)
+	var keynames FilterKeynames
+	if tr2.rcvr_base.RcvrConfig.filterSettings != nil {
+		keynames = tr2.rcvr_base.RcvrConfig.filterSettings.Keynames
+	}
+	traces := tr2.ToTraces(dl, keynames)
 
 	err := tr2.rcvr_base.TracesConsumer.ConsumeTraces(tr2.rcvr_base.ctx, traces)
 	if err != nil {