fix: support nullable end times on timesheet creation

See https://github.com/kimai/kimai/issues/3720

doc.json

@@ -4231,7 +4231,8 @@
         "end": {
           "example": "2023-01-14T05:12:43",
           "type": "string",
-          "format": "date-time"
+          "format": "date-time",
+          "x-nullable": true
         },
         "project": {
           "description": "Project ID",

models/timesheet_edit_form.go

@@ -38,7 +38,7 @@ type TimesheetEditForm struct {
 	// end
 	// Example: 2023-01-14T05:12:43
 	// Format: date-time
-	End strfmt.DateTime `json:"end,omitempty"`
+	End *strfmt.DateTime `json:"end,omitempty"`
 
 	// exported
 	Exported bool `json:"exported,omitempty"`