Alerting: Fix mathexp.NoData for ConditionsCmd (#56816) (#56820)

(cherry picked from commit 004bb7689d)

Co-authored-by: George Robinson <george.robinson@grafana.com>

pkg/expr/classic/classic.go

@@ -82,6 +82,11 @@ func (cmd *ConditionsCmd) Execute(_ context.Context, vars mathexp.Vars) (mathexp
 			var reducedNum mathexp.Number
 			var name string
 			switch v := val.(type) {
+			case mathexp.NoData:
+				// To keep this code as simple as possible we translate mathexp.NoData into a
+				// mathexp.Number with a nil value so number.GetFloat64Value() returns nil
+				reducedNum = mathexp.NewNumber("no data", nil)
+				reducedNum.SetValue(nil)
 			case mathexp.Series:
 				reducedNum = c.Reducer.Reduce(v)
 				name = v.GetName()

pkg/expr/classic/classic_test.go

@@ -326,6 +326,29 @@ func TestConditionsCmdExecute(t *testing.T) {
 		},
 		{
 			name: "single query with no data",
+			vars: mathexp.Vars{
+				"A": mathexp.Results{
+					Values: []mathexp.Value{mathexp.NoData{}.New()},
+				},
+			},
+			conditionsCmd: &ConditionsCmd{
+				Conditions: []condition{
+					{
+						InputRefID: "A",
+						Reducer:    reducer("avg"),
+						Operator:   "and",
+						Evaluator:  &thresholdEvaluator{"gt", 1},
+					},
+				},
+			},
+			resultNumber: func() mathexp.Number {
+				v := valBasedNumber(nil)
+				v.SetMeta([]EvalMatch{{Metric: "NoData"}})
+				return v
+			},
+		},
+		{
+			name: "single query with no values",
 			vars: mathexp.Vars{
 				"A": mathexp.Results{
 					Values: []mathexp.Value{},