Fix __module__ default when run in an exec

[andyljones]

Running typeguard 2.13 in an exec with an empty environment errors. This PR fixes that by mapping a f.__module__ of None to the empty string.

exec("""
from typeguard import typechecked

@typechecked
def f():
   pass

""", {})

(The root issue is that this exec-in-empty-namespace thing is what doctests like to do)

[coveralls]

Coverage decreased (-0.8%) to 85.281% when pulling 8e86f41 on andyljones:exec-fix into 8cfbe29 on agronholm:master.

[agronholm]

Thanks!