Bugzilla 42334: System properties may be none string objects

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@535312 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/Project.java

@@ -910,7 +910,9 @@ public class Project implements ResourceFactory {
         while (e.hasMoreElements()) {
             String propertyName = (String) e.nextElement();
             String value = systemP.getProperty(propertyName);
-            this.setPropertyInternal(propertyName, value);
+            if (value != null) {
+                this.setPropertyInternal(propertyName, value);
+            }
         }
     }