Unset JAVA_HOME and JDK_HOME when no java version set by asdf

[jarmo]

Currently JAVA_HOME and JDK_HOME variables are set by set-java-home.bash when Java version is specified by asdf. This might cause JAVA_HOME and JDK_HOME to be persisted even when Java version is not specified anymore (for example navigating out of directory with local .tool-versions file).

For example:

~$ echo $JAVA_HOME

~$ cd project
~/project$ cat .tool-versions 
java temurin-22.0.1+8
~/project$ echo $JAVA_HOME 
/home/user/.asdf/installs/java/temurin-22.0.1+8
~/project$ cd ..
~$ echo $JAVA_HOME 
/home/user/.asdf/installs/java/temurin-22.0.1+8

This PR clears JAVA_HOME when it is not configured via asdf to keep it in sync. Similar problem probably exists for other supported shells, but since I don't use these then I can't test any changes.