Fix empty string comparisons on option types

[cpcloud]

No description provided.

[cpcloud]

@llllllllll @kwmsmith this PR breaks the following test:

============================================================ FAILURES ============================================================
_______________________________________________________ test_scalar_coerce _______________________________________________________
blaze/expr/tests/test_scalar.py:258: in test_scalar_coerce
    assert scalar_coerce('?date', '') == None
E   assert datetime.date(2015, 11, 19) == None
E    +  where datetime.date(2015, 11, 19) = scalar_coerce('?date', '')

I think that a comparison such as t.date_column == '' should raise a TypeError. Thoughts?

[cpcloud]

Right now, t.date_column == '' would be equivalent to t.date_column == None

[llllllllll]

I think this is correct behavior. I was actually suprised that comparisons with strings coerced to datetimes at all. I am onboard with changing the test and deprecating that behavior