fix ReplaceRenderer class. Method Parse, NullReferenceException

[wangengzheng]

close #355

[wangengzheng]

The original intention of the adjustment is that when the model attribute is assumed to be null after release, it is difficult to find obvious errors in the corresponding stack

System.NullReferenceException: Object reference not set to an instance of an object.
   at FluentEmail.Core.Defaults.ReplaceRenderer.Parse[T](String template, T model, Boolean isHtml)
   at FluentEmail.Core.Defaults.ReplaceRenderer.ParseAsync[T](String template, T model, Boolean isHtml)