Membership operator with None causes typerror.

[Onager]

This query:

IfElse(
Union(Membership(
 Literal('last message repeated')
 Var('body')))
Literal('repeated')
Literal(None))

Causes a TypeError when the "body" var is None. Stacktrace:

  File "/usr/lib/python2.7/dist-packages/efilter/api.py", line 125, in apply
    results = solve.solve(query, vars).value
  File "/usr/lib/python2.7/dist-packages/efilter/dispatch.py", line 193, in __call__
    return implementation(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/efilter/transforms/solve.py", line 208, in solve_query
    return solve(query.root, vars)
  File "/usr/lib/python2.7/dist-packages/efilter/dispatch.py", line 193, in __call__
    return implementation(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/efilter/transforms/solve.py", line 403, in solve_ifelse
    if boolean.asbool(solve(condition, vars).value):
  File "/usr/lib/python2.7/dist-packages/efilter/dispatch.py", line 193, in __call__
    return implementation(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/efilter/transforms/solve.py", line 644, in solve_union
    result = solve(child, vars)
  File "/usr/lib/python2.7/dist-packages/efilter/dispatch.py", line 193, in __call__
    return implementation(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/efilter/transforms/solve.py", line 771, in solve_membership
    return Result(needle in values, ())
  File "/usr/lib/python2.7/dist-packages/efilter/transforms/solve.py", line 162, in __solve_and_destructure_repeated
    values = iter(__solve_for_repeated(expr, vars))
TypeError: 'NoneType' object is not iterable