Bugfix: model rebuild avoidance in Prism works ok when there are no undefined constants.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@4608 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/prism/Prism.java

@@ -1430,6 +1430,8 @@ public class Prism implements PrismSettingsListener
 	 */
 	public void setPRISMModelConstants(Values definedMFConstants) throws PrismLangException
 	{
+		if (currentDefinedMFConstants == null && definedMFConstants == null)
+			return;
 		if (currentDefinedMFConstants != null && currentDefinedMFConstants.equals(definedMFConstants))
 			return;