MINOR: sample: don't needlessly call c_none() in sample_fetch_as_type()

MINOR: sample: don't needlessly call c_none() in sample_fetch_as_type()

Surprisingly, while about all calls to a sample cast function carefully
avoid calling c_none(), sample_fetch_as_type() makes no effort regarding
this, while by nature, the function is most often called with an expected
output type similar to the one of the expresison. Let's add it to shorten
the most common call path.