Error in handling HierarchicalQuery with only StudyInstanceUID parameter

[Sjoerd Kerkstra]

• dicomtrolley version: 3.0.4

For the following query:
study = trolley.find_study(query=Query(StudyInstanceUID="1.2.3"))

dicomtrolley fires the wrong URL. The url will omit the StudyInstanceUID, which is wrong. The faulty code is here:

https://github.com/sjoerdk/dicomtrolley/blob/v3.0.4/dicomtrolley/qido_rs.py#L224

This code works for multi-study searchers involving additional search parameters (find all with name AB* for example). But NOT for the case above.

Fix this