Reverted the logic of CCLabelTTF::updateTexture.

In `CCLabelTTF::updateTexture` function:
The return value of '`CCTexture::initWithString`' is different on different platforms.
On android, it will always return true, while on iOS and win32, it will return false if an `empty string` was passed in.
If we return false for 'updateTexture', the display of LabelTTF will not be updated.
So even if the return value of 'initWithString' is `false`, we shouldn't return `false` for '`updateTexture`' directly.