MappedJwtClaimSetConverter#withDefaults doesn't remove claims from JWT as documented

[fguenci]

Describe the bug
As stated in Spring Security Documentation, to remove a claim from a JWT just pass a converter for the claim in MappedJwtClaimSetConverter.withDefaults() that return null.
Actually this setting doesn't remove the claim from JWT.

I think the problem is in class org.springframework.security.oauth2.core.converter.ClaimTypeConverter, method convert.

	public Map<String, Object> convert(Map<String, Object> claims) {
		if (CollectionUtils.isEmpty(claims)) {
			return claims;
		}

		Map<String, Object> result = new HashMap<>(claims);
		this.claimTypeConverters.forEach((claimName, typeConverter) -> {
			if (claims.containsKey(claimName)) {
				Object claim = claims.get(claimName);
				Object mappedClaim = typeConverter.convert(claim);
				if (mappedClaim != null) {
					result.put(claimName, mappedClaim);
				}
			}
		});

		return result;
	}

I think that the result map should contain all the mapped claims even if its value is null because null value claims are removed later.

To Reproduce
Steps to reproduce:
I want to remove the NBF claim from jwt.
To do that I set in my jwtdecoder a converter that return null for this claim:

		var jwtDecoder = NimbusJwtDecoder.withJwkSetUri(jwkSetUri).build();
		var converter = MappedJwtClaimSetConverter.withDefaults(Collections.singletonMap(JwtClaimNames.NBF, nbfClaimValue -> null));
		jwtDecoder.setClaimSetConverter(converter);

Expected behavior
The decoded JWT doen't contains the NBF claim.

[jzheaux]

I agree, @fguenci. Are you able to submit a PR with the change, including a test in MappedJwtClaimSetConverterTests that matches this use case?

[fguenci]

ok, i'll try