Fix expires_at: null in CouchDB

This issue was discovered by @Merkur39. When creating an
io.cozy.permissions for a share by link, the expires_at field was set to
null in CouchDB, even if the field was declared as omitempty. A nil
pointer to a time.Time is not considered by Go as an empty value for a
field of the type interface{}. So, we need to use the empty value of
type interface{} instead to fix the JSON marshaling.

model/permission/permissions.go

@@ -500,7 +500,14 @@ func checkSetPermissions(set Set, parent *Permission) error {
 }
 
 // CreateShareSet creates a Permission doc for sharing by link
-func CreateShareSet(db prefixer.Prefixer, parent *Permission, sourceID string, codes, shortcodes map[string]string, subdoc Permission, expiresAt *time.Time) (*Permission, error) {
+func CreateShareSet(
+	db prefixer.Prefixer,
+	parent *Permission,
+	sourceID string,
+	codes, shortcodes map[string]string,
+	subdoc Permission,
+	expiresAt interface{},
+) (*Permission, error) {
 	set := subdoc.Permissions
 	if err := checkSetPermissions(set, parent); err != nil {
 		return nil, err

web/permissions/permissions.go

@@ -135,7 +135,7 @@ func createPermission(c echo.Context) error {
 		return err
 	}
 
-	var expiresAt *time.Time
+	var expiresAt interface{}
 	if ttl != "" {
 		if d, errd := bigduration.ParseDuration(ttl); errd == nil {
 			ex := time.Now().Add(d)