fix: db index and duplicate credentials error

[hperl]

This fixes two minor bugs

• uses an improved index for getting the credential identifier from the user handle
• checks if the password hash is set before suggesting it as a credential identifier on duplicate credential errors

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[aeneasr]

Just one minor thing regarding migration

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.06%. Comparing base (c435727) to head (d335e71).
Report is 2 commits behind head on master.

Files	Patch %	Lines
identity/manager.go	77.77%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3896      +/-   ##
==========================================
+ Coverage   78.01%   78.06%   +0.04%     
==========================================
  Files         360      360              
  Lines       25257    25276      +19     
==========================================
+ Hits        19705    19732      +27     
+ Misses       4041     4034       -7     
+ Partials     1511     1510       -1     

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