Merge pull request #915 from opendatatrentino/fixNullCellToString

Fixes Cell.toString failing on null value

main/src/com/google/refine/model/Cell.java

@@ -193,6 +193,10 @@ public class Cell implements HasFields, Jsonizable {
     
     @Override
     public String toString() {
-        return value.toString();
+        if (value != null){
+            return value.toString();
+        } else {
+            return "null";
+        }
     }
 }