Support null text on paragraphs

I think this was an old feature on medium, but you can see examples of
null text on this post:

https://medium.com/message/the-joy-of-typing-fd8d091ab8ef

src/classes/markup_converter.cr

@@ -12,11 +12,12 @@ class MarkupConverter
   getter markups : Array(PostResponse::Markup)
   getter text : String
 
-  def self.convert(text : String, markups : Array(PostResponse::Markup))
+  def self.convert(text : String?, markups : Array(PostResponse::Markup))
     new(text, markups).convert
   end
 
-  def initialize(@text : String, @markups : Array(PostResponse::Markup))
+  def initialize(text : String?, @markups : Array(PostResponse::Markup))
+    @text = text || ""
   end
 
   def convert : Array(Child)

src/models/post_response.cr

@@ -32,7 +32,7 @@ class PostResponse
   end
 
   class Paragraph < Base
-    property text : String
+    property text : String?
     property type : ParagraphType
     property markups : Array(Markup)
     property iframe : IFrame?