Avoid instantiating entire dataset by getting the nbytes in an array

Using `.data` accidentally tries to load the whole lazy arrays into
memory.

Sad.

xarray/core/variable.py

@@ -403,8 +403,8 @@ def nbytes(self) -> int:
         If the underlying data array does not include ``nbytes``, estimates
         the bytes consumed based on the ``size`` and ``dtype``.
         """
-        if hasattr(self.data, "nbytes"):
-            return self.data.nbytes
+        if hasattr(self._data, "nbytes"):
+            return self._data.nbytes
         else:
             return self.size * self.dtype.itemsize