Alerting/SSE: Fix "count_non_null" reducer validation (#35451)

fixes error "reducer 'count_non_null' in condition 2 is not a valid reducer"

pkg/expr/classic/reduce.go

@@ -15,7 +15,7 @@ func (cr classicReducer) ValidReduceFunc() bool {
 	switch cr {
 	case "avg", "sum", "min", "max", "count", "last", "median":
 		return true
-	case "diff", "diff_abs", "percent_diff", "percent_diff_abs", "count_not_null":
+	case "diff", "diff_abs", "percent_diff", "percent_diff_abs", "count_non_null":
 		return true
 	}
 	return false

pkg/expr/classic/reduce_test.go

@@ -112,6 +112,7 @@ func TestReducer(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, true, tt.reducer.ValidReduceFunc())
 			num := tt.reducer.Reduce(tt.inputSeries)
 			require.Equal(t, tt.expectedNumber, num)
 		})