fix(provisioner): support empty selectors

[speedfl]

Pull Request template

Why is this PR required? What issue does it fix?:

See #285

What this PR does?:

This PR excludes empty selectors. Instead of using == nil it use len(). It also adds some new unit tests.

Does this PR require any upgrade changes?:

If the changes in this PR are manually verified, list down the scenarios covered::

Any additional information for your reviewer? :
Mention if this PR is part of any design or a continuation of previous PRs

Checklist:

• Fixes [provisioner] StatefulSet with empty selector are blocked #285
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[tiagolobocastro]

Cool, thank you!

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.54%. Comparing base (cab53c4) to head (ebef8e1).
⚠️ Report is 53 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #286      +/-   ##
===========================================
- Coverage    37.91%   37.54%   -0.38%     
===========================================
  Files           36        1      -35     
  Lines         3373      783    -2590     
===========================================
- Hits          1279      294     -985     
+ Misses        2012      480    -1532     
+ Partials        82        9      -73     

Flag	Coverage Δ
integrationtests	37.54% <ø> (?)

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[niladrih]

LGTM. Thank you for this change @speedfl!

[niladrih]

Let me know if you're ready to merge @speedfl. I'll merge this on Monday (20th Oct.) unless you confirm.

[speedfl]

I am ready let's go 🚀