chore: add NullNode case while converting values for mertrics reporting

[sanket-mundra]

Description

Please include a summary of the change, motivation and context.

Testing

Please describe the tests that you ran to verify your changes. Please summarize what did you test and what needs to be tested e.g. deployed and tested helm chart locally.

Checklist:

• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

Documentation

Make sure that you have documented corresponding changes in this repository or hypertrace docs repo if required.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.60%. Comparing base (015ee5c) to head (3fcca80).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ntstore/metric/BaseDocStoreMetricProviderImpl.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #222      +/-   ##
============================================
- Coverage     79.61%   79.60%   -0.02%     
  Complexity     1048     1048              
============================================
  Files           204      204              
  Lines          5083     5084       +1     
  Branches        416      416              
============================================
  Hits           4047     4047              
- Misses          740      741       +1     
  Partials        296      296              

Flag	Coverage Δ
integration	79.60% <0.00%> (-0.02%)	⬇️
unit	56.31% <0.00%> (-0.02%)	⬇️

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[github-actions]

Test Results

 40 files  ±0   40 suites  ±0   33s ⏱️ ±0s
246 tests ±0  246 ✅ ±0  0 💤 ±0  0 ❌ ±0 
502 runs  ±0  502 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 3fcca80. ± Comparison against base commit 015ee5c.