Fix BooleanConversionFilter bug when nil value at top level

This fixes an issue where if the top-level config hash contains a `nil`, we would break out of `each_pair`, causing the entire `execute` method to return `nil` (and therefore making Chamber return `nil` for `Chamber.env.data`).

This might not be the best fix -- I'm not sure what the intention of the original code was. Depending on the answer to that question, `next if value.nil?` might be more appropriate.