don't raise validation error if the message is empty

lib/params/base_validator.rb

@@ -89,12 +89,13 @@ module Params
       raise_error(message || "#{key} must be unique")
     end
 
-    def in_path(params, path = [])
+    def in_path(params, path = [], skip_blank: false)
       old_path = @current_path
 
       @current_path = [old_path, *path].compact_blank.map(&:to_s).join('.')
 
       param = params.dig(*path)
+      param = nil if skip_blank && param.blank?
 
       yield params.dig(*path) if param
 

lib/params/submission_create_validator.rb

@@ -49,7 +49,7 @@ module Params
       type(params, :message, Hash)
       type(params, :submitters, Array)
 
-      in_path(params, :message) do |message_params|
+      in_path(params, :message, skip_blank: true) do |message_params|
         type(message_params, :subject, String)
         type(message_params, :body, String)