warning for %%timeit empty cell

Notifies user if they run %%timeit with an empty cell block - see issue  #2862.

IPython/core/magics/execution.py

@@ -776,6 +776,10 b' python-profiler package from non-free.""")'
                                         posix=False, strict=False)
         if stmt == "" and cell is None:
             return
+        elif cell == '':
+            warn("Empty cell. Did you mean to use %timeit?")
+            return
+        
         timefunc = timeit.default_timer
         number = int(getattr(opts, "n", 0))
         repeat = int(getattr(opts, "r", timeit.default_repeat))