sqs_service.rb: fix exception when related tags sqs not configured.

Fixes this exception when Danbooru.config.aws_sqs_enabled? is true but
Danborou.config.aws_sqs_reltagcalc_url is not configured:

    ArgumentError exception raised

    missing required parameter params[:queue_url]
    app/logical/sqs_service.rb:11:in `send_message'
    app/models/tag.rb:674:in `update_related_if_outdated'
    app/models/tag.rb:698:in `related_tag_array'
    app/presenters/post_set_presenters/post.rb:49:in `related_tags_for_single'
    app/presenters/post_set_presenters/post.rb:19:in `related_tags'
    app/presenters/post_set_presenters/post.rb:8:in `initialize'
    app/logical/post_sets/post.rb:181:in `new'
    app/logical/post_sets/post.rb:181:in `presenter'
    app/views/posts/index.html.erb:14:in `_app_views_posts_index_html_erb___3235672853362939702_47243208308540'
    app/controllers/posts_controller.rb:16:in `index'

app/logical/sqs_service.rb

@@ -5,8 +5,12 @@ class SqsService
     @url = url
   end
 
+  def enabled?
+    Danbooru.config.aws_sqs_enabled? && credentials.set? && url.present?
+  end
+
   def send_message(string, options = {})
-    return unless Danbooru.config.aws_sqs_enabled?
+    return unless enabled?
 
     sqs.send_message(
       options.merge(
@@ -18,16 +22,17 @@ class SqsService
 
 private
 
+  def credentials
+    @credentials ||= Aws::Credentials.new(
+      Danbooru.config.aws_access_key_id,
+      Danbooru.config.aws_secret_access_key
+    )
+  end
+
   def sqs
-    @sqs ||= begin
-      credentials = Aws::Credentials.new(
-        Danbooru.config.aws_access_key_id,
-        Danbooru.config.aws_secret_access_key
-      )
-      Aws::SQS::Client.new(
-        credentials: credentials,
-        region: Danbooru.config.aws_sqs_region
-      )
-    end
+    @sqs ||= Aws::SQS::Client.new(
+      credentials: credentials,
+      region: Danbooru.config.aws_sqs_region
+    )
   end
 end