Remove BeanResolver Null Checks

[snowykte0426]

Resolves gh-17816.

Spring Framework updated StandardEvaluationContext.setBeanResolver
to accept a @Nullable BeanResolver
(spring-projects/spring-framework#35371, included in 6.2.11 and 7.0.x).

Since Spring Security now depends on Spring Framework 7.0.7,
the temporary NullAway workarounds around setBeanResolver
can be removed.

Changes

Removed temporary workarounds from the following locations.

Removed @SuppressWarnings("NullAway")

• DefaultHttpSecurityExpressionHandler
• reactive CurrentSecurityContextArgumentResolver
• reactive AuthenticationPrincipalArgumentResolver

Removed unnecessary null guards

• servlet AuthenticationPrincipalArgumentResolver
• servlet CurrentSecurityContextArgumentResolver
• messaging reactive CurrentSecurityContextArgumentResolver
• DefaultMessageSecurityExpressionHandler

Also removed obsolete comments referencing
spring-projects/spring-framework#35371 and cleaned up
unused imports/local variables where applicable.

Behavior

No behavior change.

StandardEvaluationContext.beanResolver defaults to null,
so calling setBeanResolver(null) is equivalent to not invoking
the setter.

Verification

• ./gradlew :spring-security-web:compileJava :spring-security-messaging:compileJava
• ./gradlew :spring-security-web:test --tests "*AuthenticationPrincipalArgumentResolver*" --tests "*CurrentSecurityContextArgumentResolver*" --tests "*DefaultHttpSecurityExpressionHandler*"
• ./gradlew :spring-security-messaging:test --tests "*CurrentSecurityContextArgumentResolver*" --tests "*DefaultMessageSecurityExpressionHandler*"
• grep -rn "35371" --include="*.java"

[snowykte0426]

A few choices I made that I'd like to confirm before this is merged:

1. Scope creep into spring-security-core. The issue body refers to "comments linking to StandardEvaluationContext.setBeanResolver should allow @Nullable BeanResolver spring-framework#35371", which strictly matches seven sites in web/ and messaging/. While verifying I noticed two more sites that implement the same workaround without the marker comment:

   • AbstractSecurityExpressionHandler#createEvaluationContext (line ~81) — same if (this.beanResolver != null) pattern.
   • DefaultMethodSecurityExpressionHandler#createEvaluationContext (line ~98) — Optional.ofNullable(getBeanResolver()).ifPresent(ctx::setBeanResolver); (same workaround, different idiom).

   I folded them into this PR since the issue title says "Remove null checks to work around setBeanResolver(BeanResolver)" and these are clearly the same workaround. Happy to split them out if you'd prefer to keep this PR strictly to the seven commented sites.

2. @SuppressWarnings("NullAway") on DefaultMethodSecurityExpressionHandler#createEvaluationContext. The suppression there is annotated // FIXME: Dataflow analysis limitation, which reads like a separate concern from #35371, so I left it in place even though the Optional workaround it covered is now gone. Let me know if you'd like it removed too.