Adding first registry in CLI erases the default one

Describe the bug

By default, registry list in CLI contains a default registry registry.quarkus.io and there is an option to add new registries. Although, if user adds a new registry, e.g. registry.quarkus.redhat.com, then the default registry will be silently removed from the list and became inaccessible.

Expected behavior

Default registry stays in the list, registries are never removed without explicit user request.

Actual behavior

Default registry is removed from the list.

How to Reproduce?

1. Install quarkus afresh or remove/rename file ~/.quarkus/config.yaml
2. Check default registry.

$ quarkus registry list
Configured Quarkus extension registries:
registry.quarkus.io

1. Check existing extensions.

$ quarkus ext list | wc -l
920

1. Add non-default registry: quarkus registry add registry.quarkus.redhat.com
2. Check registry list:

$ quarkus registry list
Configured Quarkus extension registries:
registry.quarkus.redhat.com #registry.quarkus.io is not in the list
(Read from ~/.quarkus/config.yaml)

1. Check existing extensions.

$ quarkus ext list | wc -l
170 # at least some extensions are not accessible any more

Output of uname -a or ver

5.14.17-201.fc34.x86_64

Output of java -version

Java version: 11.0.13, vendor: Eclipse Adoptium

GraalVM version (if different from Java)

No response

Quarkus version or git rev

2.5.1.Final

Build tool (ie. output of mvnw --version or gradlew --version)

Apache Maven 3.8.4

Additional information

No response

[gsmet]

I think we could just add a warning when executing quarkus registry add registry.quarkus.redhat.com if there is no registry set.

[maxandersen]

The config file can be present without a registry in it.

The suggestion is to print a warning if no registry is already set. i.e. the list of registries are empty in the config file.