disabled sourcemap output in case no source files are present

Author: mrnateriver

lib/loader.js

@@ -108,14 +108,15 @@ module.exports = function(content, map) {
 				return "\" + escape(require(" + loaderUtils.stringifyRequest(this, urlRequest) + ")) + \"";
 			}.bind(this));
 		}
-		
+
 		var exportJs = compileExports(result, importItemMatcher.bind(this), camelCaseKeys);
 		if (exportJs) {
 			exportJs = "exports.locals = " + exportJs + ";";
 		}
 
 		var moduleJs;
-		if(sourceMap && result.map) {
+		// do not generate source maps for empty sources
+		if(sourceMap && result.map && result.map.sources.length) {
 			// add a SourceMap
 			map = result.map;
 			if(map.sources) {