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

[metteo]

Affected version

3.2.0

Bug description

Old toolchain goal was warning about empty config: ToolchainMojo.java:77:

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?