MappedJwtClaimSetConverter calls Converter#convert with null

Describe the bug
Documentation of org.springframework.core.convert.converter.Converter#convert clearly says "never null":

* @param source the source object to convert, which must be an instance of {@code S} (never {@code null})

But lines 156-158 of org.springframework.security.oauth2.jwt.MappedJwtClaimSetConverter haven't got null-checks in them:

Object claim = claims.get(claimName); // claim is nullable
Object mappedClaim = converter.convert(claim); // convert accepts non-null !!!
mappedClaims.compute(claimName, (key, value) -> mappedClaim);

This results in NullPointerException when accepting JWT without particular claims (which of them that haven't got custom converters for them).

What's interesting is that default converters (e.g. convertInstant) has null-checks INSIDE of them:

private static Instant convertInstant(Object source) {
    if (source == null) {
        return null;
    }
// ... snip ...
}

To Reproduce
Set custom ClaimSetConverter in NimbusJwtDecoder:

decoder.setClaimSetConverter(MappedJwtClaimSetConverter
    .withDefaults(Map.of(JwtClaimNames.SUB, source -> {
        return "Custom conversion result";
    })));

Then make a request with JWT without "sub" claim.

Expected behavior

Conversion is not called for absent claims.

Start in oauth2/oauth2-jose/src/main/java/org/springframework/security/oauth2/jwt/MappedJwtClaimSetConverter.java at lines 156-158, then follow the NimbusJwtDecoder claim-set converter entry point. Reproduce the case with a custom converter and a JWT missing the configured claim. Done means absent claims no longer invoke the converter or cause a NullPointerException.