[JENKINS-48738] Fix NPE when setting JDK

[MarkEWaite]

Fix null pointer exception when setting JDK

See the original proposed change at a15d15a

Confirmed with interactive testing that the null pointer exception is resolved and that the configuration slicing plugin can change the value of the JDK that is defined on a Freestyle job. I changed between jdk-8 and jdk-20.

The plugin cannot reset the definition of the JDK to the default because that causes a null pointer exception in AbstractProject as reported by the original bug report JENKINS-48738

Avoid the null pointer exception by ignoring the attempt to reset the JDK value to default.

Thanks to @Evildethow and @ahertier for implementation

Testing done

Interactive testing of freestyle project with Jenkins 2.401. I configured two JDK tool values, jdk-8 and jdk-20, then assigned one of the values to the job. The configuration slicing plugin with this change is able to change the configuration from jdk-8 to jdk-20 and from jdk-20 to jdk-8. The attempt to change JDK to an empty string is sliently ignored rather than generating a null pointer exception.

AbstractProject does not seem to have a method that will allow the JDK of a project to be reset to the default value. The user interface is able to make that change, but apparently it makes the change without passing a null to setJDK().

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