Check debugger_restart_minutes for nil in config.rb

In node's ```config.rb```, the comment for ```$debugger_restart_minutes``` says: "Set to nil to disable this feature (restarting debugger)". What happens is if you do set this to nil, ```$debugger_restart_minutes``` will be evaluated this way in ```configuration.rb```:

```ruby
if($debugger_restart_minutes < 5 )
	print_warning( "Warning, you have set the debugger to restart every #{$debugger_restart_minutes} minutes, The Grinder Server will see this node as inactive unless you use a value of more than 5 minutes." )
end
```

When you do nil < 5, you should get:

```
NoMethodError: undefined method `<' for nil:NilClass
```

So this PR fixes that.

I looked around a bit more to see if the same problem exists elsewhere, didn't see anything. ```grinder.rb``` uses ```$debugger_restart_minutes``` too for the sleep function, but nil condition is already checked in there before calculation..