Fix NPE when ./gradlew run without --data-dir (elastic#50421) (elastic#50425)

Author: pugnascotia

buildSrc/src/main/java/org/elasticsearch/gradle/testclusters/RunTask.java

@@ -3,6 +3,7 @@
 import org.gradle.api.logging.Logger;
 import org.gradle.api.logging.Logging;
 import org.gradle.api.tasks.Input;
+import org.gradle.api.tasks.Optional;
 import org.gradle.api.tasks.TaskAction;
 import org.gradle.api.tasks.options.Option;
 
@@ -49,7 +50,9 @@ public void setDataDir(String dataDirStr) {
     }
 
     @Input
+    @Optional
     public String getDataDir() {
+        if (dataDir == null) { return null;}
         return dataDir.toString();
     }