Change Credential sfSubjectNode from required to optional

[Bronek]

High Level Overview of Change

Fix object template for Credential object.

Context of Change

Field sfSubjectNode is not populated by CredentialCreate in self-issued credentials. Rather than fixup the Credentials already on the ledger, we can in this case safely change the object template for this field from soeREQUIRED to soeOPTIONAL.

We know that this is safe (despite no amendment) because:

• object template is never a part of serialized data and never used in hashing or object signing
• we have done this before, changing sfAuthorize in DepositPreauth in XLS-70d Credentials #5103

If we don't do it, the only safe way to refer to sfSubjectNode will be by comparing sfIssuer against sfSubject beforehand, as currently done in CredentialDelete transaction. Although in this specific context this is fine, such approach in general is unsafe.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.5%. Comparing base (2bf77cc) to head (887d525).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5936     +/-   ##
=========================================
- Coverage     79.5%   78.5%   -1.0%     
=========================================
  Files          817     817             
  Lines        72198   69017   -3181     
  Branches      8293    8274     -19     
=========================================
- Hits         57392   54199   -3193     
- Misses       14806   14818     +12     

Files with missing lines	Coverage Δ
include/xrpl/protocol/detail/ledger_entries.macro	100.0% <ø> (ø)

... and 492 files with indirect coverage changes

🚀 New features to boost your workflow:

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