fix: hash_on field and limitation

[Baoyuantop]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

In the Upstream creation or configuration page, when Hash on selected consumer, the Key field is not required.

Related issues

fix #2012

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[netlify]

✔️ Deploy Preview for apisix-dashboard ready!

🔨 Explore the source changes: b50c821

🔍 Inspect the deploy log: https://app.netlify.com/sites/apisix-dashboard/deploys/610f40054aeefe0008593823

😎 Browse the preview: https://deploy-preview-2034--apisix-dashboard.netlify.app/

[codecov-commenter]

Codecov Report

Merging #2034 (b50c821) into master (5198854) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2034      +/-   ##
==========================================
- Coverage   65.74%   65.69%   -0.05%     
==========================================
  Files         121      121              
  Lines        3202     3204       +2     
  Branches      776      778       +2     
==========================================
  Hits         2105     2105              
- Misses       1097     1099       +2     

Flag	Coverage Δ
frontend-e2e-test	65.69% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
web/src/components/Upstream/components/Type.tsx	100.00% <100.00%> (ø)
web/src/helpers.tsx	68.85% <0.00%> (-3.28%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5198854...b50c821. Read the comment docs.