Don't warn about 'sandbox' when 'bind' is empty

Empty object as a value of 'bind' module option will not produce any
require() calls, but still is useful for modules exporting its stuff via
setting of modules's `this` object properties.
Merge pull request #173 from leonidborisenko/sandbox-with-empty-bind-object

Don't warn about 'sandbox' when 'bind' is empty