fix(contrib/cloud.google.com/go/pubsub.v1): don't panic on checking f…

…or span links

contrib/cloud.google.com/go/pubsub.v1/tracing.go

@@ -118,7 +118,7 @@ func TraceReceiveFunc(s Subscription, opts ...Option) func(ctx context.Context,
 			opts = append(opts, tracer.Measured())
 		}
 		// If there are span links as a result of context extraction, add them as a StartSpanOption
-		if parentSpanCtx.SpanLinks() != nil {
+		if parentSpanCtx != nil && parentSpanCtx.SpanLinks() != nil {
 			opts = append(opts, tracer.WithSpanLinks(parentSpanCtx.SpanLinks()))
 		}
 		span, ctx := tracer.StartSpanFromContext(ctx, cfg.receiveSpanName, opts...)