[FSTORE-1693] Null values in feature group causes built-in transformation function label encoder to Fail

[manu-sj]

Issue:
Null values in the column used for a label encoder causes it to cash with the error .

Root Cause:
Hopsworks statistics retturns None as a unique element if it is present in a column. This cause the label_encoder to fail because the None cannot be used as key in a dictionary for label value lookup.

Fix Done:
Ignore None while creating a dictionary for label value look up. Null values are not encoded and retruned as None itself so that the user can perform any imputation strategy.

JIRA Issue: https://hopsworks.atlassian.net/browse/FSTORE-1693

Priority for Review: -

Related PRs: -

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)