Handling empty view in sort_by_similarity()

[brimoor]

Handles a previously uncaught edge case where view.sort_by_similarity() would raise an error if view is empty.

import fiftyone as fo
import fiftyone.brain as fob
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart", max_samples=5).clone()
results = fob.compute_similarity(dataset, brain_key="image_sim")

# Previously would raise an error; now succeeds
view = dataset.limit(0).sort_by_similarity(dataset.first().id)

Note that other methods like find_duplicates() and find_unique() will still raise an error on empty views, but I'm not going to do anything about that right now because:

• The methods won't output anything useful in such cases anyway
• The methods aren't currently used by the App, where fatal errors are less acceptable
• The error message is informative

results.use_view(dataset.limit(0))

results.find_duplicates(thresh=0.1)

ValueError: Found array with 0 sample(s) (shape=(0, 0)) while a minimum of 1 is required by NearestNeighbors.