🐛 Fix issue #135 - return correct null value.

The AbstractObjectAttributeDescription.getString() method returns
`String.valueOf((T) data[id])` even if `data[id]` is null. This returns
the String "null" when a float_or_null value is null, which in turn
causes Float.parseFloat() at line 90 in RestUtilities.addData() to
throw a java.lang.NumberFormatException: For input string: "null".

This commit checks for a null value and returns either a non-null
value or null.

CoreGraph/src/au/gov/asd/tac/constellation/graph/attribute/AbstractObjectAttributeDescription.java

@@ -89,7 +89,7 @@ public void setCapacity(final int capacity) {
 
     @Override
     public String getString(final int id) {
-        return String.valueOf((T) data[id]);
+        return data[id]!=null ? String.valueOf((T) data[id]) : null;
     }
 
     @Override