fix: Add missing result check in DecentralizedIdentityService

[bscholtes1A]

What this PR changes/adds

Add assertion of Result returned by CredentialsVerifier invokation.

Why it does that

Prevent NullPointerException when CredentialsVerifier invokation fails.

Further notes

List other areas of code that have changed but are not necessarily linked to the main feature. This could be method signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)

[codecov-commenter]

Codecov Report

Base: 63.24% // Head: 63.88% // Increases project coverage by +0.64% 🎉

Coverage data is based on head (cac754f) compared to base (2d40b2f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2005      +/-   ##
==========================================
+ Coverage   63.24%   63.88%   +0.64%     
==========================================
  Files         789      779      -10     
  Lines       16655    16513     -142     
  Branches     1084     1077       -7     
==========================================
+ Hits        10533    10549      +16     
+ Misses       5670     5513     -157     
+ Partials      452      451       -1     

Impacted Files	Coverage Δ
...nnector/identity/DecentralizedIdentityService.java	82.92% <100.00%> (+1.34%)	⬆️
.../spi/types/domain/transfer/ResourceDefinition.java	84.21% <0.00%> (-15.79%)	⬇️
...connector/iam/oauth2/core/Oauth2Configuration.java	74.28% <0.00%> (-10.72%)	⬇️
...sferprocessstore/InMemoryTransferProcessStore.java	85.00% <0.00%> (-10.00%)	⬇️
...aceconnector/common/reflection/ReflectionUtil.java	79.78% <0.00%> (-7.72%)	⬇️
...r/ids/token/validation/rule/IdsValidationRule.java	65.00% <0.00%> (-1.67%)	⬇️
...eclipse/dataspaceconnector/spi/iam/ClaimToken.java	0.00% <0.00%> (ø)
...nnector/api/auth/BasicAuthenticationExtension.java	31.57% <0.00%> (ø)
...paceconnector/iam/oauth2/core/Oauth2Extension.java	0.00% <0.00%> (ø)
...injection/lifecycle/ExtensionLifecycleManager.java	100.00% <0.00%> (ø)
... and 52 more

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