proper argument for cursor.execute (for sa engine)

[NotSoSuper]

For the SQLAlchemy engine, the _distill_params function in sa/connection.py returns an empty list [] when no arguments are passed.

This results in [] passed to cursor.execute as the args argument causing if check
https://github.com/aio-libs/aiomysql/blob/master/aiomysql/cursors.py#L237
to string format an empty list, erroring with:

File "/usr/local/lib/python3.6/site-packages/aiomysql/utils.py", line 75, in __await__
    resp = yield from self._coro
  File "/usr/local/lib/python3.6/site-packages/aiomysql/sa/connection.py", line 77, in _execute
    yield from cursor.execute(query, dp)
  File "/usr/local/lib/python3.6/site-packages/aiomysql/cursors.py", line 238, in execute
    query = query % self._escape_args(args, conn)
TypeError: not enough arguments for format string

[codecov]

Codecov Report

Merging #239 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #239   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files           9        9           
  Lines        1157     1157           
  Branches      172      172           
=======================================
  Hits         1075     1075           
  Misses         53       53           
  Partials       29       29

Impacted Files	Coverage Δ
aiomysql/sa/connection.py	92.3% <100%> (ø)	⬆️

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[asvetlov]

Please add a unittest for the fix.

[jettify]

thanks a lot!

[jettify]

new version on pypi https://pypi.python.org/pypi/aiomysql/0.0.12