fix @maps serialization bug

if @maps is empty, it returns {"maps":[]}, instead of [] like we expect
on the frontend.

This commit fixes this issue

app/controllers/maps_controller.rb

@@ -19,7 +19,7 @@ class MapsController < ApplicationController
         redirect_to(root_url) && return if authenticated?
         respond_with(@maps, @user)
       end
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end
 
@@ -33,7 +33,7 @@ class MapsController < ApplicationController
 
     respond_to do |format|
       format.html { respond_with(@maps, @user) }
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end
 
@@ -51,7 +51,7 @@ class MapsController < ApplicationController
 
     respond_to do |format|
       format.html { respond_with(@maps, @user) }
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end
 
@@ -69,7 +69,7 @@ class MapsController < ApplicationController
 
     respond_to do |format|
       format.html { respond_with(@maps, @user) }
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end
 
@@ -88,7 +88,7 @@ class MapsController < ApplicationController
 
     respond_to do |format|
       format.html { respond_with(@maps, @user) }
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end
 
@@ -101,7 +101,7 @@ class MapsController < ApplicationController
 
     respond_to do |format|
       format.html { respond_with(@maps, @user) }
-      format.json { render json: @maps }
+      format.json { render json: @maps.to_json }
     end
   end