HHH-17994 defensive check for the case when classNames wasn't set

(cherry picked from commit 08127f2)

tooling/hibernate-enhance-maven-plugin/src/main/java/org/hibernate/orm/tooling/maven/MavenEnhancePlugin.java

@@ -112,7 +112,7 @@ public void execute() throws MojoExecutionException, MojoFailureException {
 		}
 
 		List<String> classesToEnhance = new ArrayList<>();
-		if(classNames.length() >= 1) {
+		if(classNames != null && classNames.length() >= 1) {
 			classesToEnhance = Arrays.asList(classNames.split(","));
 		}