Thank you for writing.
In the last release we introduced fixes and improvements to the Null value support in filtering operation. I have tested your filtering scenario and I found that the introduced changes caused a regression on the filtering operation when empty string value is used. The issue is already resolved and fix will be included in the Q1 2013 SP1 release, planed to be available for download by the end of the month.
Thank you for the report. Your Telerik points have been updated.
All the best,
the Telerik team