Fix CommandContext throwing an exception for multiple spaces.

When using quotes for multiword arguments, CommandContext will throw a `StringIndexOutOfBoundsException` if the String checked is empty:

![Example](https://f.cloud.github.com/assets/1849924/735565/51ca5fce-e2f8-11e2-9c94-43c176f94501.png)

This can be avoided by adding `if (arg2.length == 0) continue;` inside the for-loop. However, the spaces the user entered are lost (can be fixed by also adding `build.append(' ');` in the if block).

PS: I apologise for completely forgetting about my other pull request. About to fix that now, as well.