Fix ledger_entry crash on invalid request

[Bronek]

High Level Overview of Change

RPC ledger_entry is not performing sufficient checks on input for deposit_preauth, which causes a crash.

Context of Change

In #5103 we have added support for authorized_credentials array to deposit_preauth objects for the ledger_entry RCP request. The checks for the correct inputs in this authorized_credentials array were not exhaustive enough. This PR is fixing it, alongside with fixing the unit tests to improve the coverage.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.9%. Comparing base (9d58f11) to head (cba54a0).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5189     +/-   ##
=========================================
- Coverage     77.9%   77.9%   -0.1%     
=========================================
  Files          782     782             
  Lines        66616   66619      +3     
  Branches      8161    8158      -3     
=========================================
- Hits         51902   51870     -32     
- Misses       14714   14749     +35     

Files with missing lines	Coverage Δ
src/xrpld/rpc/handlers/LedgerEntry.cpp	79.8% <100.0%> (+0.2%)	⬆️

... and 11 files with indirect coverage changes