Creation of an empty DataArray

I’d like to create an empty DataArray, i.e., one with only NA values. The docstring of DataArray says that data=None is allowed, if a dataset argument is provided. However, the docstring doesn’t say anything about a dataset argument.

1. I think there’s a bug in the docstring
2. I’d like to pass data=None and get a DataArray with the coords/dims set up properly (as defined by the coords and dims kwargs), but with a values array of NA.

[aldanor]

It might also make sense then to implement all numpy-like constructors for DataArray, plus the empty(), which is typically faster for larger arrays:

• .full() (kind of what’s suggested here)
• .ones()
• .zeros()
• .empty()

This should be trivial to implement.

[shoyer]

Fixed the doc string for now, but this would still be a nice feature to add at some point.