Add specialization for compile-time zero-sized dense assignment.

In the current `dense_assignment_loop` implementations, if the
destination's inner or outer size is zero at compile time and if the kernel
involves a product, we currently get a compile error (#2080).  This is
triggered by attempting to multiply a non-existent row by a column (or
vice-versa).

To address this, we add a specialization for zero-sized assignments
(`AllAtOnceTraversal`) which evaluates to a no-op. We also add a static
check to ensure the size is in-fact zero. This now seems to be the only
existing use of `AllAtOnceTraversal`.

Fixes #2080.

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