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'
    >
    >
    >

PR-URL: https://github.com/nodejs/io.js/pull/2163


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>