TSQL conditions raise error on empty values

[goodmami]

Executing a TSQL regex-match condition on a column with empty values can raise an error as these values are cast to None:

$ delphin select 'i-id where error ~ "lexical gap"' mrs/
Traceback (most recent call last):
[...]
  File "/home/mwg/repos/pydelphin/delphin/tsql.py", line 422, in func
    return re.search(body[1], value)
  File "/usr/local/lib/python3.6/re.py", line 182, in search
    return _compile(pattern, flags).search(string)
TypeError: expected string or bytes-like object

The problem is that value (in tsql.py) is None. A similar error occurs when the query's datatype doesn't match the column's datatype (see #261) but this problem with None also occurs even when they do match.

Edit: changed the title and text because it occurs with other operators and datatypes (e.g, doing an inequality comparison on a missing :date value).

[goodmami]

Note that this behavior may be affected by #258, e.g., if the meaning of tsdb.cast() changes.