Enable Cat of Zero Element Vec (#1623)

* Return 0.U for asUInt of a zero-element Seq

Add a condition to SeqUtils.asUInt to have it return an unspecified
width 0.U when applied to an empty sequence. This enables the ability
to do a Cat of a zero-element sequence.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

* Test elaboration of Cat on zero-element Seq

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>