slices: update Int.Scan() to parse empty slices

Previously, Int.Scan() returned the slice '[0]' when reading an empty
slice of ints from the database. This is a bug, as the output should
also be empty. This patch fixes the bug and adds regression tests.
Additionally, the code is updated to percolate up the errors from
integer to string conversion, which was previously swallowed.

slices/int.go

@@ -27,8 +27,10 @@ func (i *Int) Scan(src interface{}) error {
 	default:
 		return fmt.Errorf("scan source was not []byte nor string but %T", src)
 	}
-	*i = strToInt(str)
-	return nil
+
+	v, err := strToInt(str)
+	*i = v
+	return err
 }
 
 // Value implements the driver.Valuer interface.
@@ -56,12 +58,23 @@ func (i *Int) UnmarshalText(text []byte) error {
 	return nil
 }
 
-func strToInt(s string) []int {
+func strToInt(s string) ([]int, error) {
 	r := strings.Trim(s, "{}")
 	a := make([]int, 0, 10)
-	for _, t := range strings.Split(r, ",") {
-		i, _ := strconv.Atoi(t)
+
+	split := strings.Split(r, ",")
+	// Split returns [""] when splitting the empty string.
+	if len(split) == 1 && split[0] == "" {
+		return a, nil
+	}
+
+	for _, t := range split {
+		i, err := strconv.Atoi(t)
+		if err != nil {
+			return nil, err
+		}
 		a = append(a, i)
 	}
-	return a
+
+	return a, nil
 }

slices/int_test.go

@@ -0,0 +1,30 @@
+package slices
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_Int_Scan(t *testing.T) {
+	r := require.New(t)
+	t.Run("empty slice", func(t *testing.T) {
+		in := "{}"
+		v := &Int{}
+		r.NoError(v.Scan(in))
+		r.Len(*v, 0)
+	})
+
+	t.Run("non-empty slice", func(t *testing.T) {
+		in := "{44,55}"
+		v := &Int{}
+		r.NoError(v.Scan(in))
+		r.Equal([]int(*v), []int{44, 55})
+	})
+
+	t.Run("invalid entry", func(t *testing.T) {
+		in := "{44,word}"
+		v := &Int{}
+		r.Error(v.Scan(in))
+	})
+}