tracer: improve coverage

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

tracer/context_test.go

@@ -5,6 +5,24 @@ import (
 	"testing"
 )
 
+func TestFromNilContext(t *testing.T) {
+	// nolint: staticcheck
+	c, ok := FromContext(nil)
+	if ok || c != nil {
+		t.Fatal("FromContext not works")
+	}
+}
+
+func TestNewNilContext(t *testing.T) {
+	// nolint: staticcheck
+	ctx := NewContext(nil, NewTracer())
+
+	c, ok := FromContext(ctx)
+	if c == nil || !ok {
+		t.Fatal("NewContext not works")
+	}
+}
+
 func TestFromContext(t *testing.T) {
 	ctx := context.WithValue(context.TODO(), tracerKey{}, NewTracer())