repl: fix passing an empty line inserting "undefined" into the buffer

There was a possiblity of insering the string "undefined" into the repl's
command buffer, which would cause interesting results while evaluating.

lib/repl.js

@@ -883,6 +883,7 @@ function trimWhitespace(cmd) {
   if (matches && matches.length === 2) {
     return matches[1];
   }
+  return '';
 }
 
 

test/simple/test-repl.js

@@ -152,7 +152,9 @@ function error_test() {
     { client: client_unix, send: 'npm install foobar',
       expect: expect_npm },
     { client: client_unix, send: '(function () {\n\nreturn 1;\n})()',
-      expect: '1' }
+      expect: '1' },
+    { client: client_unix, send: '{\n\na: 1\n}',
+      expect: '{ a: 1 }' }
   ]);
 }