Handling the case where key is empty in dpath search

In our project, we had empty key that was causing dpath search to fail all together. since, the line             
```if issubclass(k.__class__, (basestring)) and skip and k[0] == '+':```
checking for k[0] without validating k is empty or not.

Just skipping for the case where key is empty. Nothing else is touch upon.


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