fix: add default timestamp field

[SuZhou-Joe]

Description

We are integrating ppl_visualization in Notebook page and found that if the timestamp field of a ppl metadata is empty string, the ppl_visualization is unable to launch and render in Notebook page, but is able to render in visualization detail page.

Before:

Unable to render in Notebook page:

able to render in visualization page:

After

Able to render in Notebook page:

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (cdbcfa4) 51.57% compared to head (b511e16) 51.56%.

Files	Patch %	Lines
public/components/custom_panels/helpers/utils.tsx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1329      +/-   ##
==========================================
- Coverage   51.57%   51.56%   -0.01%     
==========================================
  Files         305      305              
  Lines       10651    10652       +1     
  Branches     2793     2794       +1     
==========================================
  Hits         5493     5493              
- Misses       5112     5113       +1     
  Partials       46       46              

Flag	Coverage Δ
dashboards-observability	51.56% <0.00%> (-0.01%)	⬇️

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