Fixed material contentEquals crash when technique is null.

git-svn-id: https://jmonkeyengine.googlecode.com/svn/trunk@9458 75d07b2b-3a1a-0410-a2c5-0572b91ccdca

engine/src/core/com/jme3/material/Material.java

@@ -252,8 +252,8 @@ public class Material implements CloneableSmartAsset, Cloneable, Savable {
             // E.g. if user chose custom technique for one material but 
             // uses default technique for other material, the materials 
             // are not equal.
-            String thisDefName = this.technique != null ? this.technique.getDef().getName() : null;
-            String otherDefName = other.technique != null ? other.technique.getDef().getName() : null;
+            String thisDefName = this.technique != null ? this.technique.getDef().getName() : "";
+            String otherDefName = other.technique != null ? other.technique.getDef().getName() : "";
             if (!thisDefName.equals(otherDefName)) {
                 return false;
             }