Convert lack of value to an empty string, not nil.

Pros:
first - if we send "?admin" query then we are able to make conditions like this:
```
if params[:admin]
 ...
end
```
w/ nil we needed to check existance of key using has_key? and that is not nice.

second - we fix some CVE, known and possibly unknown.

Cons:
I don't see anything bad. Seems @tenderlove @steveklabnik don't see either.