Early exit from trigger if value is null or empty

In accordance with code review on
https://github.com/duckduckgo/zeroclickinfo-spice/pull/984
Change TfL API call to use HTTPS
Disable caching of TfL API responses

We want realtime status information, so we must disable the default
caching of API responses.
Add some more triggers based on top autocomplete

Based on suggestions in the duckduckgo.com autocomplete, I've added:

- xxxx line london
- xxxx line london underground
- xxxx line london tube
- xxxx line underground
- xxxx line tube