Enable the use of SyncedVariantReader with no intervals

Specify "all intervals" by passing in an empty string.
This is due to a quirk (bug?) in htslib.  Multiple files only work when `require_index` is set, and this is set automatically when `bcf_sr_set_regions` is called.

Not sure whether the best approach is to add an explanatory comment here or make a change in htslib.