Add null strategy for runners

* This is the default strategy used when the runner management is disabled. It might be replaced with a generic Docker strategy in the future (without pooling). For now, it allows normal "operation" of CodeOcean without any runner management. However, as no runner system is configured, no command can be executed.

app/models/runner.rb

@@ -11,10 +11,12 @@ class Runner < ApplicationRecord
   attr_accessor :strategy
 
   def self.strategy_class
-    @strategy_class ||= begin
-      strategy_name = CodeOcean::Config.new(:code_ocean).read[:runner_management][:strategy]
-      "runner/strategy/#{strategy_name}".camelize.constantize
-    end
+    @strategy_class ||= if Runner.management_active?
+                          strategy_name = CodeOcean::Config.new(:code_ocean).read[:runner_management][:strategy]
+                          "runner/strategy/#{strategy_name}".camelize.constantize
+                        else
+                          Runner::Strategy::Null
+                        end
   end
 
   def self.management_active?