Fix bilby pipe generation compatibility

What?

This update changes the behaviour of the data-dict parameter in the bilby configuration file to resolve a bug introduced by changes in the most recent version of bilby_pipe.

How?

The bilby_pipe update that fixed issues with updated htcondor broke the integration with asimov. After the update, it will only look for data to download if data-dict is not None. As currently implemented in asimov, data-dict can be empty but never none. The change that broke it can be seen here.

Who?

This MR was prepared by @tomasz.baka.

When?

Scheduled for inclusion in 0.5.4.