Don't increment execution_count on empty cells. +test.

IPython/core/interactiveshell.py

@@ -2118,6 +2118,9 b' class InteractiveShell(Configurable, Magic):'
           history. For user code calling back into IPython's machinery, this
           should be set to False.
         """
+        if not cell.strip():
+            return
+        
         raw_cell = cell
         with self.builtin_trap:
             cell = self.prefilter_manager.prefilter_lines(cell)

IPython/core/tests/test_interactiveshell.py

@@ -41,7 +41,9 b' class InteractiveShellTestCase(unittest.TestCase):'
         """Just make sure we don't get a horrible error with a blank
         cell of input. Yes, I did overlook that."""
         ip = get_ipython()
+        old_xc = ip.execution_count
         ip.run_cell('')
+        self.assertEquals(ip.execution_count, old_xc)
 
     def test_run_cell_multiline(self):
         """Multi-block, multi-line cells must execute correctly.