allow missing source in SourceMap webpack#954

Author: sokra

lib/ModuleFilenameHelpers.js

@@ -47,6 +47,7 @@ function asRegExp(test) {
 }
 
 ModuleFilenameHelpers.createFilename = function createFilename(module, moduleFilenameTemplate, requestShortener) {
+	if(!module) module = "";
 	if(typeof module === "string") {
 		var shortIdentifier = requestShortener.shorten(module);
 		var identifier = shortIdentifier;

lib/SourceMapDevToolPlugin.js

@@ -91,8 +91,8 @@ SourceMapDevToolPlugin.prototype.apply = function(compiler) {
 			}, function(ai, bi) {
 				var a = allModules[ai];
 				var b = allModules[bi];
-				a = typeof a === "string" ? a : a.identifier();
-				b = typeof b === "string" ? b : b.identifier();
+				a = !a ? "" : typeof a === "string" ? a : a.identifier();
+				b = !b ? "" : typeof b === "string" ? b : b.identifier();
 				return a.length - b.length;
 			});
 			allModuleFilenames = ModuleFilenameHelpers.replaceDuplicates(allModuleFilenames, function(filename, i, n) {