[JENKINS-48738] Fix JDK configuration NPE (#203)

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 https://issues.jenkins.io/browse/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

src/main/java/configurationslicing/jdk/JdkSlicer.java

@@ -64,7 +64,9 @@ public boolean setValues(AbstractProject item, List<String> set) {
             JDK oldJdk = item.getJDK();
             if (!equals(oldJdk, jdk)) {
 	            try {
-	                item.setJDK(jdk);
+	                if (jdk != null) {
+                            item.setJDK(jdk);
+                        }
 	                return true;
 	            } catch (IOException e) {
 	                e.printStackTrace();