fix: refactor internal API to prevent panics

[zepatrik]

I noticed a nil-pointer dereference panic in github.com/ory/kratos/selfservice/strategy/code.(*Strategy).handleVerificationError

The problem really is that the internal API use(d) pointers where it was not necessary, and lacked nil checks elsewhere. This change adds nil checks, and also improves the internal API to avoid pointers when not strictly needed.

[zepatrik]

This could panic.

[zepatrik]

This could also panic.

[codecov]

Codecov Report

Attention: Patch coverage is 61.65049% with 79 lines in your changes missing coverage. Please review.

Project coverage is 78.37%. Comparing base (955bd8f) to head (fd6fffe).
Report is 2 commits behind head on master.

Files	Patch %	Lines
selfservice/strategy/lookup/settings.go	59.25%	4 Missing and 7 partials ⚠️
selfservice/strategy/code/strategy_verification.go	44.44%	10 Missing ⚠️
selfservice/strategy/link/strategy_verification.go	52.38%	10 Missing ⚠️
...fservice/strategy/profile/two_step_registration.go	20.00%	7 Missing and 1 partial ⚠️
selfservice/strategy/webauthn/registration.go	56.25%	7 Missing ⚠️
selfservice/strategy/totp/settings.go	72.72%	4 Missing and 2 partials ⚠️
selfservice/strategy/idfirst/strategy_login.go	37.50%	5 Missing ⚠️
selfservice/strategy/password/registration.go	54.54%	5 Missing ⚠️
selfservice/strategy/passkey/passkey_settings.go	55.55%	4 Missing ⚠️
selfservice/strategy/password/login.go	66.66%	3 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4028      +/-   ##
==========================================
+ Coverage   78.35%   78.37%   +0.02%     
==========================================
  Files         370      370              
  Lines       26113    26120       +7     
==========================================
+ Hits        20460    20472      +12     
+ Misses       4094     4090       -4     
+ Partials     1559     1558       -1     

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