fix(cpp): remove UB when reading unset builder vertex id

[Sober7135]

Reason for this PR

#846

What changes are included in this PR?

• Represent builder::Vertex id as std::optional<IdType> instead of an uninitialized scalar
• Make GetId() fail explicitly when the id is unset, instead of reading uninitialized memory
• Remove the stored empty_ flag from builder::Vertex
• Derive Empty() from properties_.empty() so vertex emptiness is no longer tracked by redundant mutable state
• Add/adjust tests to cover:
  • default-constructed vertex with unset id
  • SetId() / constructor-initialized id behavior
  • Empty() behavior after removing the stored empty_ field

Are these changes tested?

Yes

Are there any user-facing changes?

Yes

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.87%. Comparing base (d535a43) to head (8255be0).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #892      +/-   ##
============================================
+ Coverage     79.81%   79.87%   +0.05%     
  Complexity      615      615              
============================================
  Files            93       93              
  Lines         10296    10296              
  Branches       1055     1055              
============================================
+ Hits           8218     8224       +6     
+ Misses         1838     1832       -6     
  Partials        240      240              

Flag	Coverage Δ
cpp	70.91% <100.00%> (+0.11%)	⬆️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.