Support NullDB adapter (used in Docker packaging) in DBPreflightCheck initializer

lib/zammad/application/initializer/db_preflight_check.rb

@@ -1,6 +1,7 @@
 require 'zammad/application/initializer/db_preflight_check/base'
 require 'zammad/application/initializer/db_preflight_check/mysql2'
 require 'zammad/application/initializer/db_preflight_check/postgresql'
+require 'zammad/application/initializer/db_preflight_check/nulldb'
 
 module Zammad
   class Application

lib/zammad/application/initializer/db_preflight_check/nulldb.rb

@@ -0,0 +1,12 @@
+module Zammad
+  class Application
+    class Initializer
+      module DBPreflightCheck
+        module Nulldb
+          # no-op
+          def self.perform; end
+        end
+      end
+    end
+  end
+end