cxxrtl: fix handling of 0-bit variables in `vcd_writer.sample()`.

[jfng]

Before this commit, vcd_writer.sample() emits vector values for 0-bit variables.

For example, the following RTLIL:

module \top
  wire width 0 \foo
end

would result in the following VCD:

$timescale 1 us $end
$var wire 0 ! foo $end
$enddefinitions $end
#0
b !

However, according to IEEE 1364-2001, the syntax for vector values is:

After this commit, foo is no longer sampled in the VCD:

$timescale 1 us $end
$var wire 0 ! foo $end
$enddefinitions $end
#0

[jfng]

Actually, it may be better to emit a scalar value of 0 instead. If a VCD variable has no samples, GTKWave gives it the type "real" and the value "nan", which is confusing.

[whitequark]

Actually, it may be better to emit a scalar value of 0 instead.

Yeah, sounds reasonable.

[jfng]

I changed it to emit a vector value of 0 instead (eg. b0 !).

Now, both Surfer and GTKWave show a 0. Otherwise, GTKWave would show an "X" for a scalar value of a variable that isn't 1-bit.