Ignores the cache if required field exists but is falsy

[wildhart]

The return value of _getField() is usually defined by:

		if (!doc[field[i]]) {
			return; // === undefined
		}

and then the existance of the field is determined by:

if (typeof _getField(cache, item) === "undefined") {
  innocent = false;
}}

This will cause the cache to be missed if a required field exists but is falsy. E.g.:

doc = {admin: false}; // or null fails as well
field = 'admin';
_getField(doc, field); // undefined, the cache will be ignored :-(