Allow undef handlers and subscribers

This commit fixes the error message: `undefined method `sort' for
nil:NilClass`. This error message can be seen when either the handlers
or subscribers parameters are passed in as  `undef` in the check.pp
manifest.

The PRs that looks like they introduced this behavior are:
#285
#303

lib/puppet/type/sensu_check.rb

@@ -45,7 +45,8 @@ def insync?(is)
   newproperty(:handlers, :array_matching => :all) do
     desc "List of handlers that responds to this check"
     def insync?(is)
-      is.sort == should.sort
+      return is.sort == should.sort if is.is_a?(Array) && should.is_a?(Array)
+      is == should
     end
   end
 
@@ -81,7 +82,8 @@ def insync?(is)
   newproperty(:subscribers, :array_matching => :all) do
     desc "Who is subscribed to this check"
     def insync?(is)
-      is.sort == should.sort
+      return is.sort == should.sort if is.is_a?(Array) && should.is_a?(Array)
+      is == should
     end
   end