If file path is an empty string, return false directly in CCFileUtils::isFileExist.
Fixing a bug in CCFileUtilsIOS::isFileExist().
Merge branch 'master' of https://github.com/cocos2d/cocos2d-x into lua-bytecode-test
Merge pull request #2457 from dumganhar/master

Fixing a bug in CCFileUtilsIOS::isFileExist(). Improving performance of isFileExist for all platforms.