repl: better empty line handling

In REPL, if we try to evaluate an empty line, we get `undefined`.

    > process.version
    'v2.3.4'
    >
    undefined
    >
    undefined
    >

This patch prevents `undefined` from printing if the string is empty.

    > process.version
    'v2.3.5-pre'
    >
    >
    >

lib/repl.js

@@ -357,7 +357,7 @@ function REPLServer(prompt,
       }
     }
 
-    if (!skipCatchall) {
+    if (!skipCatchall && (cmd || (!cmd && self.bufferedCommand))) {
       var evalCmd = self.bufferedCommand + cmd;
       if (/^\s*\{/.test(evalCmd) && /\}\s*$/.test(evalCmd)) {
         // It's confusing for `{ a : 1 }` to be interpreted as a block

test/parallel/test-repl.js

@@ -225,6 +225,13 @@ function error_test() {
     // using REPL command "help" within a string literal should still work
     { client: client_unix, send: '\'thefourth\\\n.help\neye\'',
       expect: /'thefourtheye'/ },
+    // empty lines in the REPL should be allowed
+    { client: client_unix, send: '\n\r\n\r\n',
+      expect: prompt_unix + prompt_unix + prompt_unix },
+    // empty lines in the string literals should not affect the string
+    { client: client_unix, send: '\'the\\\n\\\nfourtheye\'\n',
+      expect: prompt_multiline + prompt_multiline +
+              '\'thefourtheye\'\n' + prompt_unix },
   ]);
 }