Transferring empty outer lists at lower complexities

## Background

https://abs-tudelft.github.io/tydi/specification/physical.html#last-signal-description

> [C<4] It is illegal to assert a `last` bit for dimension j without also asserting the `last` bits for dimensions j′
> [C<4] It is illegal to assert the last bit for dimension 0 when the respective data lane is inactive, except for empty sequences.

The first rule suggests that at C<4, and (as an example) D=3, it is not possible to assert `last="100"` or `last="110"`, or `last="010"`).

## Issue

This means that at C<4, it is illegal to transfer empty outer lists, which in turn means that Stream complexity has an effect on the _kinds of data_ transferred.

For instance, the example given in the last signal description for C≥8, D=2
`["Hello", "World"], ["Tydi", "is", "nice"], [""], []`
makes use of an empty outer list (the last element, `[]`). This requires asserting `last="10"`.

## Suggested Fix

It is not clear whether this is intentional. However, I am of the opinion that Stream properties should not affect the kind of data which can be transferred.

As such, I recommend amending the first rule in the Background section of this issue to also include an exception for empty sequences.