Add and apply Style/RedundantBegin rubocop rule

We're about to add code which might fall into the `RedundantBegin`
category, so we're adding the rule in order to prevent that.

app/controllers/graphql_controller.rb

@@ -9,25 +9,23 @@ class GraphqlController < ApplicationController
   class QueryStringError < StandardError; end
 
   def execute
-    begin
-      raise GraphqlController::QueryStringError if query_string.nil?
+    raise GraphqlController::QueryStringError if query_string.nil?
 
-      result = ConsulSchema.execute(
-        query_string,
-        variables: prepare_variables,
-        context: {},
-        operation_name: params[:operationName]
-      )
-      render json: result
-    rescue GraphqlController::QueryStringError
-      render json: { message: "Query string not present" }, status: :bad_request
-    rescue JSON::ParserError
-      render json: { message: "Error parsing JSON" }, status: :bad_request
-    rescue GraphQL::ParseError
-      render json: { message: "Query string is not valid JSON" }, status: :bad_request
-    rescue ArgumentError => e
-      render json: { message: e.message }, status: :bad_request
-    end
+    result = ConsulSchema.execute(
+      query_string,
+      variables: prepare_variables,
+      context: {},
+      operation_name: params[:operationName]
+    )
+    render json: result
+  rescue GraphqlController::QueryStringError
+    render json: { message: "Query string not present" }, status: :bad_request
+  rescue JSON::ParserError
+    render json: { message: "Error parsing JSON" }, status: :bad_request
+  rescue GraphQL::ParseError
+    render json: { message: "Query string is not valid JSON" }, status: :bad_request
+  rescue ArgumentError => e
+    render json: { message: e.message }, status: :bad_request
   end
 
   private