SSE: Add noData type (#51973)

When there is a single frame with no fields (e.g. splunk datasource) SSE errors when trying to figure out the data type. This frame needs to exist since this is where the executedQueryString metadata exists.

This adds a new return type to SSE to represent no data, so the original frame with its metadata can still be maintained.

pkg/expr/nodes.go

@@ -237,7 +237,7 @@ func (dn *DSNode) Execute(ctx context.Context, vars mathexp.Vars, s *Service) (m
 		}
 
 		dataSource := dn.datasource.Type
-		if isAllFrameVectors(dataSource, qr.Frames) {
+		if isAllFrameVectors(dataSource, qr.Frames) { // Prometheus Specific Handling
 			vals, err = framesToNumbers(qr.Frames)
 			if err != nil {
 				return mathexp.Results{}, fmt.Errorf("failed to read frames as numbers: %w", err)
@@ -247,6 +247,12 @@ func (dn *DSNode) Execute(ctx context.Context, vars mathexp.Vars, s *Service) (m
 
 		if len(qr.Frames) == 1 {
 			frame := qr.Frames[0]
+			// Handle Untyped NoData
+			if len(frame.Fields) == 0 {
+				return mathexp.Results{Values: mathexp.Values{mathexp.NoData{Frame: frame}}}, nil
+			}
+
+			// Handle Numeric Table
 			if frame.TimeSeriesSchema().Type == data.TimeSeriesTypeNot && isNumberTable(frame) {
 				logger.Debug("expression datasource query (numberSet)", "query", refID)
 				numberSet, err := extractNumberSet(frame)