JwtDecoderProviderConfigurationUtils incorrectly handles issuer URI

[bodograumann]

Describe the bug
JwtDecoderProviderConfigurationUtils use UriComponentsBuilder to modify a URI in oidc, oidcRfc8414 and oauth.
Due to a known issue in UriComponents builder (spring-projects/spring-framework#27774), this produces invalid URIs in the cases where the URI contains an authority, but no host.
These still come up in practice though, e.g. see the following partial backtrace:

Caused by: java.lang.IllegalArgumentException: Unable to resolve the Configuration with the provided Issuer of "http://elated_sutherland:8080/auth/realms/my-realm"
        at org.springframework.security.oauth2.jwt.JwtDecoderProviderConfigurationUtils.getConfiguration(JwtDecoderProviderConfigurationUtils.java:178)
        at org.springframework.security.oauth2.jwt.JwtDecoderProviderConfigurationUtils.getConfigurationForIssuerLocation(JwtDecoderProviderConfigurationUtils.java:90)
        at org.springframework.security.oauth2.jwt.NimbusJwtDecoder.lambda$withIssuerLocation$2(NimbusJwtDecoder.java:226)
        at org.springframework.security.oauth2.jwt.NimbusJwtDecoder$JwkSetUriJwtDecoderBuilder.processor(NimbusJwtDecoder.java:389)
        at org.springframework.security.oauth2.jwt.NimbusJwtDecoder$JwkSetUriJwtDecoderBuilder.build(NimbusJwtDecoder.java:405)
        at org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerJwtConfiguration$JwtDecoderConfiguration.lambda$jwtDecoderByIssuerUri$3(OAuth2ResourceServerJwtConfiguration.java:159)
        at org.springframework.security.oauth2.jwt.SupplierJwtDecoder.lambda$new$0(SupplierJwtDecoder.java:36)
        ... 88 common frames omitted
Caused by: org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http:/auth/realms/my-realm/.well-known/openid-configuration": Failed to select a proxy
        at org.springframework.web.client.RestTemplate.createResourceAccessException(RestTemplate.java:915)
        at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:895)
        at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:740)
        at org.springframework.security.oauth2.jwt.JwtDecoderProviderConfigurationUtils.getConfiguration(JwtDecoderProviderConfigurationUtils.java:167)
        ... 94 common frames omitted

To Reproduce

import org.springframework.web.util.UriComponentsBuilder;

import java.net.URI;
import java.util.Collections;

String rawUrl = "http://elated_sutherland:8080/auth/realms/my-realm";
URI uri = URI.create(rawUrl);
System.out.println(uri.getAuthority());
System.out.println(uri.getHost());
URI newUrl = UriComponentsBuilder.fromUri(uri)
  .replacePath(uri.getPath() + "/.well-known/openid-configuration")
  .build(Collections.emptyMap());
System.out.println(newUrl);

shows what happens. It outputs:

elated_sutherland:8080
null
http:/auth/realms/my-realm/.well-known/openid-configuration

Expected behavior
The correct oidc URI should be generated: http://elated_sutherland:8080/auth/realms/my-realm/.well-known/openid-configuration.