Fix bug in subset[!] when handling no conditions case

[bkamins]

This PR fixes a situation when we can only detect that no conditions are passed after processing them. Example of wrong behavior (fixed by this PR):

julia>     df = DataFrame(a=1:2)
2×1 DataFrame
 Row │ a
     │ Int64
─────┼───────
   1 │     1
   2 │     2

julia> subset(df, Not(:a) .=> ByRow(Returns(true)))
ERROR: AssertionError: !(isempty(conditions))

[bkamins]

Thank you!