JAVA_HOME not set yields an ambiguous method selection error

If JAVAFX_HOME is not set, we go through the logic which hunts for the JavaFX JAR in JAVA_HOME.
But if JAVA_HOME is not set, the javaHome variable contains null.
And when calling the File constructor, Groovy won't know if we're calling File(File, String) or File(String, String).
So it'd be good to do a check to see if the JAVA_HOME variable is indeed properly set to fail gracefully.