[FSTORE-1606] Allow ´entries´ to be None while retrieving feature vectors from a feature view with only on-demand features

[manu-sj]

This PR fixes an issue in which dummy data needs to be provided using entires to the function get_feature_vector while retrieving feature vectors from a feature view with only on-demand features.

Issue:
User have to always provide entires in the function get_feature_vector when reading feature vectors from a feature view.

In the use case in which a feature view contains only on-demand features, all features are computed while retrieving feature vectors using provided request parameter. Hence entries is not a mandatory parameter.

Fix Done:
Made entries into an optional parameter in cases where the feature view contains only on-demand features.

JIRA Issue: https://hopsworks.atlassian.net/browse/FSTORE-1606

Priority for Review: -

Related PRs: https://github.com/logicalclocks/loadtest/pull/485

How Has This Been Tested?

• Unit Tests
• Integration Tests
• Manual Tests on VM

Checklist For The Assigned Reviewer:

- [ ] Checked if merge conflicts with master exist
- [ ] Checked if stylechecks for Java and Python pass
- [ ] Checked if all docstrings were added and/or updated appropriately
- [ ] Ran spellcheck on docstring
- [ ] Checked if guides & concepts need to be updated
- [ ] Checked if naming conventions for parameters and variables were followed
- [ ] Checked if private methods are properly declared and used
- [ ] Checked if hard-to-understand areas of code are commented
- [ ] Checked if tests are effective
- [ ] Built and deployed changes on dev VM and tested manually
- [x] (Checked if all type annotations were added and/or updated appropriately)

[bubriks]

Looks good, just minor feedback.