repl: dont throw ENOENT on NODE_REPL_HISTORY_FILE

If you have no history file written to disk, but the environment
variable set, `fs.readFileSync` will throw an ENOENT error,
but there's nothing to convert. The converter should ignore
ENOENT on that `fs.readFileSync` call.

Fixes: https://github.com/nodejs/node/issues/2449
PR-URL: https://github.com/nodejs/node/pull/2451


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>