Fix: improve handling of missing data and simplify loan calculations

[MrRoudyk]

This pull request includes changes to improve the handling of missing data and simplify the codebase across multiple files. The most important changes include updating the calculation of collateral and debt statistics to handle missing interest rate models and simplifying the processing of loan entities.

Improvements to handling missing data:

• apps/dashboard_app/helpers/protocol_stats.py: Updated the get_collateral_stats and get_debt_stats functions to handle missing interest rate models by using default values. [1] [2]
• apps/legacy_app/src/helpers.py: Added error logging in the get_prices function to handle cases where token information is not found.

Codebase simplification:

• apps/legacy_app/src/state.py: Simplified the set_loan_entities method by removing unnecessary JSON decoding and directly using the loan entity data.
• apps/legacy_app/src/types.py: Simplified the compute_collateral_usd and compute_debt_usd methods by using the walrus operator to handle underlying addresses and checking their presence in prices. [1] [2]
• apps/shared/loan_entity.py: Updated the compute_collateral_usd and compute_debt_usd methods to handle missing interest rate models by using default values and simplified the processing of collateral and debt items. [1] [2]