Add "empty" option to enum field renderers, if field allows empty value

tailbone/forms/renderers/common.py

@@ -209,6 +209,8 @@ class EnumFieldRenderer(SelectFieldRenderer):
 
     def render(self, **kwargs):
         opts = [(self.enumeration[x], x) for x in self.enumeration]
+        if not self.field.is_required():
+            opts.insert(0, self.field._null_option)
         return SelectFieldRenderer.render(self, opts, **kwargs)