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_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)
 		})