Add null check for authentication token in JwtAuthenticationProvider

[chanbinme]

Summary

Add Assert.notNull validation in JwtAuthenticationProvider.authenticate() to ensure the authentication token returned by jwtAuthenticationConverter is not null.

Problem

While the JwtAuthenticationConverter.convert() method typically returns a valid AbstractAuthenticationToken, there's no explicit null check before accessing the token's properties. This could potentially lead to a NullPointerException if:

• Custom converter implementations return null
• Unexpected edge cases occur during token conversion
• External factors affect the conversion process

Changes

• Added Assert.notNull(token, "token cannot be null") validation
• Added comprehensive test to verify the null check behavior

Testing

• New test passes and verifies proper exception handling
• All existing tests continue to pass
• No breaking changes to existing functionality

[rwinch]

Thank you for the pull request @chanbinme! I've pushed a fix to the checkstyle and as soon as this build finishes, this will automatically be merged.

In the future, you can catch any errors using ./gradlew format && ./gradlew check as outlined in CONTRIBUTING

[chanbinme]

Thank you so much for fixing the checkstyle issue and for the helpful guidance, @rwinch ! I'll make sure to use ./gradlew format && ./gradlew check before submitting future PRs. Really appreciate your time and support! 🙏