Check if it's not NONE too

So that we don't have to check it again in somewhere else,
and we don't really need to know if it's presented as NONE

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7345/diffs#note_20123999

app/finders/issuable_finder.rb

@@ -166,20 +166,20 @@ class IssuableFinder
   end
 
   def assignee_id?
-    params[:assignee_id].present?
+    params[:assignee_id].present? && params[:assignee_id] != NONE
   end
 
   def assignee_username?
-    params[:assignee_username].present?
+    params[:assignee_username].present? && params[:assignee_username] != NONE
   end
 
   def assignee
     return @assignee if defined?(@assignee)
 
     @assignee =
-      if assignee_id? && params[:assignee_id] != NONE
+      if assignee_id?
         User.find(params[:assignee_id])
-      elsif assignee_username? && params[:assignee_username] != NONE
+      elsif assignee_username?
         User.find_by(username: params[:assignee_username])
       else
         nil
@@ -187,11 +187,11 @@ class IssuableFinder
   end
 
   def author_id?
-    params[:author_id].present?
+    params[:author_id].present? && params[:author_id] != NONE
   end
 
   def author_username?
-    params[:author_username].present?
+    params[:author_username].present? && params[:author_username] != NONE
   end
 
   def author