Fixing issue #38 AttributeError 'NoneType' object has no attribute 'j…

…son_string' (#41)

* Bump to Quill Version 1.3.7

Bump to Quill Version 1.3.7 - This version of Quill fixes Quill Vuln quilljs/quill#2438

Here is the change commit to fix the vuln in Quill quilljs/quill#2439

The Vuln is described here: https://ossindex.sonatype.org/vuln/d96c07dd-81f9-41f6-b2bd-531143bcaeab

* Adding JS/CSS include instructions from README.md

Resolves issue [#33](#33)

* adding None return for json_string

django_quill/forms.py

@@ -14,4 +14,4 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     def prepare_value(self, value):
-        return value.json_string
+        return value.json_string if value else None