Optional strings should return nil, not empty string

[schneiderderek]

Fixes an issue for xpaths using optional strings where the returned
value would be an empty string ("") instead of nil.

[doughsay]

Hey there, I need this fix for a project I'm trying to use sweet_xml for. Any reason we can't merge this?

[stevedomin]

Hi! Any chance to get this merged?

[nathany-copia]

Any chance of getting this (or similar) in?

Right now strings don't behave as described by the README:

"If you use the optional modifier o together with a soft cast modifier (uppercase), then the value is set to nil"

[Shakadak]

Hi, no:

• This is not backward compatible: there are valid cases resulting in an empty string or list
• This bypasses the casting mechanism instead of correcting it, which is why the added test that should not convert to anything still converts to nil
• The casting mechanism is ambiguous because of how xmerl_xpath is currently used. It seems something can be done on that front, but it is much more involved than the change in this PR, and it would mean creating a new API (probably an entirely new lib) because of our backward compatibility requirement.
• The transform_fun allows to do the same as this PR