New select-jdk-toolchain goal should warn about empty config

### Affected version

3.2.0

### Bug description

Old `toolchain` goal was warning about empty config: [ToolchainMojo.java:77](https://github.com/apache/maven-toolchains-plugin/blob/7318c8874d46417065139bd8a6ede47511526799/src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java#L77):

`getLog().warn("No toolchains requirements configured.");`

Maybe the new goal should do something similar. Right now it just silently exits.
Found it after mixing old goal configuration with the new select- goal.

Maybe it could be beneficial to parse old configuration in the new goal as a fallback?

Start with the referenced ToolchainMojo.java warning and locate the implementation of the new select-jdk-toolchain goal. Compare how each goal handles an empty configuration, then determine whether the fix should only warn or also consider the suggested legacy-configuration fallback. Done means the silent exit is addressed consistently with the issue’s intended behavior.

Written by the indexing model from the issue text.