with `spring-security-oatuh2-jose-7.1.0` a `jwt` with `loa` value `null` always returns an error

Describe the bug

With spring-security-oauth2-jose-7.0.5 it is possible to have a loa with value null in a given jwt. For machine to machine communication, it makes no sense to set the loa, as no real user is present.

Implementation with spring-security-oauth2-jose-7.0.5:

	@Override
	public OAuth2TokenValidatorResult validate(Jwt token) {
		Assert.notNull(token, "token cannot be null");
		T claimValue = token.getClaim(this.claim);
		if (this.test.test(claimValue)) {
			return OAuth2TokenValidatorResult.success();
		}
		this.logger.debug(this.error.getDescription());
		return OAuth2TokenValidatorResult.failure(this.error);
	}

A loa with null value is forwarded to the configured claim validator, e.g. something like..

    @Bean
    JwtClaimValidator<String> loaClaimValidator() {
        return new JwtClaimValidator<>("loa", new Predicate<String>() {
            @Override
            public boolean test(final String loaClaimValue) {      
                return loaClaimValue == null;
            }
        });
    }

..configured within the WebSecurityConfig.

Implementation with spring-security-oauth2-jose-7.1.0:

	@Override
	public OAuth2TokenValidatorResult validate(Jwt token) {
		Assert.notNull(token, "token cannot be null");
		T claimValue = token.getClaim(this.claim);
		if (claimValue != null) {
			if (this.test.test(claimValue)) {
				return OAuth2TokenValidatorResult.success();
			}
		}
		this.logger.debug(this.error.getDescription());
		return OAuth2TokenValidatorResult.failure(this.error);
	}

A loa with value null returns always an error, the custom validator is ignored.

To Reproduce

1. Create a jwt without a value for loa.
2. Perform a request against an endpoint that validates the jwt.
3. The custom provided JwtClaimValidator is NOT called.
4. OAuth2TokenValidatorResult.failure(this.error) is returned.

Expected behavior

1. Create a jwt without a value for loa.
2. Perform a request against an endpoint that validates the jwt.
3. The custom provided JwtClaimValidator is called.
4. OAuth2TokenValidatorResult.success() validator is returned, if custom validator returns true.

Start at JwtClaimValidator.validate, using the issue's 7.0.5 and 7.1.0 implementations as the behavioral comparison. Add or update coverage for a JWT whose loa claim is null, verifying that the configured validator is called and can return success.