e.g. `$displayh(8'ha)` won't have a padding set, because it just gets
`lzero` set instead by `compute_required_decimal_places`.
It also doesn't have a width. In this case, we can just fill in a dummy
(unused) padding. Either space or zero would work, but space is a bit
more distinct given the width field follows.
Also omit writing the width if it's zero. This makes the emitted ilang
a little cleaner in places; `{8:> h0u}` is the output for this example,
now. The other possible extreme would be `{8:>00h0u}`.
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