Phlare: Fix panic on empty pprof profile (#64888)

pkg/tsdb/phlare/query.go

@@ -374,6 +374,8 @@ func treeToNestedSetDataFrame(tree *ProfileTree, profileTypeID string) *data.Fra
 	labelField := NewEnumField("label", nil)
 	fileNameField := NewEnumField("fileName", nil)
 
+	// Tree can be nil if profile was empty, we can still send empty frame in that case
+	if tree != nil {
 		walkTree(tree, func(tree *ProfileTree) {
 			levelField.Append(int64(tree.Level))
 			valueField.Append(tree.Value)
@@ -384,6 +386,7 @@ func treeToNestedSetDataFrame(tree *ProfileTree, profileTypeID string) *data.Fra
 			labelField.Append(tree.Function.FunctionName)
 			fileNameField.Append(tree.Function.FileName)
 		})
+	}
 
 	frame.Fields = append(frame.Fields, labelField.GetField(), fileNameField.GetField())
 	return frame

pkg/tsdb/phlare/query_test.go

@@ -109,6 +109,7 @@ func makeDataQuery() *backend.DataQuery {
 }
 
 func Test_treeToNestedDataFrame(t *testing.T) {
+	t.Run("sample profile tree", func(t *testing.T) {
 		tree := &ProfileTree{
 			Value: 100, Level: 0, Self: 1, Function: &Function{FunctionName: "root"}, Nodes: []*ProfileTree{
 				{
@@ -145,6 +146,13 @@ func Test_treeToNestedDataFrame(t *testing.T) {
 				data.NewField("label", nil, []int64{0, 1, 2, 3}).SetConfig(labelConfig),
 				data.NewField("fileName", nil, []int64{0, 1, 2, 3}).SetConfig(filenameConfig),
 			}, frame.Fields)
+	})
+
+	t.Run("nil profile tree", func(t *testing.T) {
+		frame := treeToNestedSetDataFrame(nil, "memory:alloc_objects:count:space:bytes")
+		require.Equal(t, 6, len(frame.Fields))
+		require.Equal(t, 0, frame.Fields[0].Len())
+	})
 }
 
 var fooProfile = &googlev1.Profile{