Do not fetch structural variants data when there are no applicable samples

[dippindots]

Fix cBioPortal/cbioportal#8920

Cause:

• This happens when you select a study list that contains structural variant profiles, and then you use a custom sample list to select samples from studies that don’t contain structural variant profiles, then our frontend will try to fetch structural variants with an empty sample list. Then we will get an error because we sent an empty sample list.

Solution:

• Do not fetch structural variants data when there are no applicable samples

Test:
https://deploy-preview-4089--cbioportalfrontend.netlify.app/results?plots_horz_selection=%7B%7D&plots_vert_selection=%7B%7D&plots_coloring_selection=%7B%7D&tab_index=tab_visualize&Action=Submit&session_id=6149d140e6ebe01a6ee32224