Do not call sprintf with zero argument

[hekike]

Calling sprintf is expensive, do not call when zero arguments are passed.
In some scenarios, an application needs to create hundreds of VError(s).

To test the change's effect:

for(var i = 0; i < 10000; i++) {
        new mod_verror.VError('Foo')
}

On my 2017 MBP 3.5Ghz, I see a 27% performance improvement after the change.

Before:

[hekike]

#63
https://cr.joyent.us/#/c/5299/