Prevent "research" from generating matches if search string is zero-length

With this code:

```
matches = []
editor.research('', lambda m: matches.append(m.span(0)))
print(len(matches))
```

I was semi-surprised to see a non-zero result printed.

Any chance that the plugin could trap on this situation and not match anything?
Same for ".search()" and ".replace()" and ".rereplace()" as well, of course.

Obviously I can do that trapping as well, before calling.