Fix stack chart issue when there is no data for one area

[DaoDaoNoCode]

JIRA: https://issues.redhat.com/browse/RHOAIENG-11048

Description

This is a known issue in Victory chart, and I am taking the workaround as suggested by the PF team here patternfly/patternfly-react#10978 (comment). If there is no data, we render it as [null] instead of [], which fixes the issue.

How Has This Been Tested?

I met some problems with generating the real data points, but you can go to the cypress test server and check the test case Serving Chart Shows Data

Way to compare:

1. Run cypress:server and cypress:open:mock in 2 terminals
2. Add it.only to the Serving Chart Shows Data test so we can check the screenshot
3. Go to modelMetrics test to check if it renders incorrectly
4. Run cypress:server:build and re-run step 1-3
5. You can see the data renders correctly

Test Impact

Update the test cases to see the mocked data better.

Request review criteria:

Self checklist (all need to be checked):

• The developer has manually tested the changes and verified that the changes work
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has added tests or explained why testing cannot be added (unit or cypress tests for related changes)

If you have UI changes:

• Included any necessary screenshots or gifs if it was a UI change.
• Included tags to the UX team if it was a UI/UX change.

After the PR is posted & before it merges:

• The developer has tested their solution on a cluster by using the image produced by the PR to main

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.47%. Comparing base (6a6a03a) to head (77ec5e0).
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3229   +/-   ##
=======================================
  Coverage   85.47%   85.47%           
=======================================
  Files        1279     1279           
  Lines       28161    28162    +1     
  Branches     7525     7526    +1     
=======================================
+ Hits        24071    24072    +1     
  Misses       4090     4090           

Files with missing lines	Coverage Δ
...ages/modelServing/screens/metrics/MetricsChart.tsx	88.63% <100.00%> (+0.13%)	⬆️

... and 4 files with indirect coverage changes

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[Gkrumbach07]

/lgtm
/approve

this looks good to me

[openshift-ci]

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