`retrieveFile` interprets empty string as "no response"

[cspotcode]

node-source-map-support/source-map-support.js

Line 147 in 817f84b

var retrieveFile = handlerExec(sharedData.retrieveFileHandlers, sharedData.internalRetrieveFileHandlers);

node-source-map-support/source-map-support.js

Lines 129 to 135 in 817f84b

	function handlerExec(list, internalList) {
	return function(arg) {
	for (var i = 0; i < list.length; i++) {
	var ret = list[i](arg);
	if (ret) {
	return ret;
	}

If a file is empty, retrieveFile can return empty string. This is falsey, and today is erroneously interpreted as "no response," causing subsequent retrieveFile handlers to be invoked.

We can change retrieveFile to allow returning undefined and fix the logic to interpret empty string as empty file.

Note: Is a breaking change.