test: Add test to verify that null cells are displayed properly in spatial embeddings

[seve]

Related bug: #858

This will require modifying the test dataset to have null values in spatial embeddings.

This ticket was originally drafted before the issue #858 was modified based on our findings on expected behavior.

Based on the actual fix deployed to address the issue, the test should verify the following behavior:

• Move from spatial to non-spatial embedding
• Expect to see less cells available on embedding compare to spatial
• Move back to spatial
• Expect to see all cells available
• Expect to not see subset or unsubset buttons available

[tihuan]

Thanks for creating all the tickets, Seve!!