Fix getting hold and credit balance of user

[olethanh]

fetch_balance_of_address and fetch_credit_balance_of_address always returned 0, which prevented allocating a new instance.

Analysis:
This was a regression introduced in #834
Function checked the balance and credit_balance field presence using hasattr() however they are dictionary fields, not properties on the object.

Solution:
Rewrite using get()

[codecov]

Codecov Report

❌ Patch coverage is 76.08696% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.84%. Comparing base (c79a8cc) to head (af47a2d).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/orchestrator/payment.py	26.66%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #835      +/-   ##
==========================================
- Coverage   64.84%   64.84%   -0.01%     
==========================================
  Files          87       88       +1     
  Lines        8095     8160      +65     
  Branches      723      734      +11     
==========================================
+ Hits         5249     5291      +42     
- Misses       2620     2640      +20     
- Partials      226      229       +3     

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