workaround ST2 segfault on empty completion entry

It seems Sublime Text 2 (build 2221) doesn't really like it when the completion list contains empty strings and segfaults. This originally happened to me when trying to complete arguments for timedelta.
Steps to reproduce:
 - use Sublime Text 2 (build 2221, ie. the newest release) with the newest SublimeJEDI plugin
 - edit a python module
 - import timedelta:  `from datetime import timedelta`
 - enter `timedelta(ho` (as in hour, but minute and the rest might also work) and wait for the completion dropdown to appear
 - ST2 crashes with a segmentation fault

Here's a minimal plugin that crashes ST2 for me:
```python
import sublime_plugin

class ExampleEventListener(sublime_plugin.EventListener):
    def on_query_completions(self, view, prefix, locations):
        return [(u'', u'foo')]
```

I tried it on two systems, both were 64-bit ubuntu.