ParseAddressList: return empty list if "" is provided

Go 1.15 handles "" in the address parser as a non error case, returning
an empty list.
Prior versions returned an error, which is not what we want.

Reported-by: anianz <a.ziegler@cioplenu.de>
Tested-by: anianz <a.ziegler@cioplenu.de>

lib/format/format.go

@@ -21,6 +21,12 @@ func ParseAddress(address string) (*models.Address, error) {
 }
 
 func ParseAddressList(s string) ([]*models.Address, error) {
+	if len(s) == 0 {
+		// workaround for go versions < 1.15
+		// 1.15 returns an empty list if "" is provided as input, prior versions
+		// return an error which is not what we want
+		return nil, nil
+	}
 	parser := gomail.AddressParser{
 		&mime.WordDecoder{message.CharsetReader},
 	}