Only initialize non-default struct values when creating an Issues struct.

[lgarron]

e.g.

Issues{Errors: []string{"First Error", "Second Error"}}

instead of

Issues{
  Errors:   []string{"First Error", "Second Error"},
  Warnings: []string{},
}

[lgarron]

I looked into this, and it wouldn't save use especially much code. Since these are mostly pasted from the output of (issues Issues) GoString(), they also don't take longer to write.

So, I'd rather leave these in, as Explicit is Better Than Implicit.