Update transformations to check for None value

[IgnaceBleukx]

Fixes #828; not sure if this is the best way to do it, but seems like the most obvious for now.

[tias]

It doesn't feel the right way, but haven't further looked into it.

[IgnaceBleukx]

Now that I think about it, probably the right way of doing this, is to let the expressions decide which arguments should recusively be transformed.
E.g., each expression would get a get_expr_args and update_expr_args function, which returns the arguments that could be an expression. This will have a few advantages: for e.g., Table, we won't ever recurse into the table, even if the array contains subexpression (and thus has_subexpr will be True).
For Cumulative, we could just return [start,dur,height,capacity] if the end variable is None.