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.

For example, lazy.js [does this](https://github.com/dtao/lazy.js/blob/476ed78f83a33759dbc883327b9d40cc2b390b0c/lazy.js#L5930). In the linked code, `context` is a parameter of IIFE creating whole code scope, so `context` is the `this` passed to IIFE, i.e. object that could be set by LMD's `bind` option.

As a sidenote: lazy.js tries also to use `exports`, but makes one wrong assumption, so it doesn't work with LMD.