warnings for empty cells with %%sx, %%system, or %%!
warnings for empty cells with %%sx, %%system, or %%!

Warns the user if they run a %%sx derivative with an empty cell - issue #2862. This is not perfect since, from within sx, there's no way to to tell which of sx/system/! were issued and we have to be generic.

ipython/ipython,ipython/ipython