Allow new `hyperSpec` object without wavelengths

This is https://github.com/cbeleites/hyperSpec/issues/296

***

Suggested:

```r
new("hyperSpec", data = data.frame(c = 1:5))
```

this works:

```r
new("hyperSpec",
     wavelength = numeric(0),
     spc = matrix(NA_real_, ncol = 0, nrow = 5), 
     data = data.frame(c = 1:5)
)
```
