sql/expression: handle null values in arithmetic expressions (#760)

sql/expression: handle null values in arithmetic expressions

Author: ajnavarro

Diff for: engine_test.go

@@ -1226,6 +1226,10 @@ var queries = []struct {
 		`SELECT CASE i WHEN 1 THEN i ELSE NULL END FROM mytable`,
 		[]sql.Row{{int64(1)}, {nil}, {nil}},
 	},
+	{
+		`SELECT (NULL+1)`,
+		[]sql.Row{{nil}},
+	},
 }
 
 func TestQueries(t *testing.T) {

Diff for: sql/expression/arithmetic.go

@@ -159,6 +159,10 @@ func (a *Arithmetic) Eval(ctx *sql.Context, row sql.Row) (interface{}, error) {
 		return nil, err
 	}
 
+	if lval == nil || rval == nil {
+		return nil, nil
+	}
+
 	lval, rval, err = a.convertLeftRight(lval, rval)
 	if err != nil {
 		return nil, err