Fix label on unnamed data array

[jsignell]

Closes #666

This resolves the MRE in the original issue. I didn't quite get around to adding a test though 😬

import xarray as xr
import numpy as np
import hvplot.xarray

ny, nx = (30, 20)
x = np.linspace(0, 1, nx)
y = np.linspace(0, 2, ny)
coords={'y': ('y', y), 'x': ('x', x)}
foo = xr.DataArray(np.random.rand(ny, nx), dims=['y', 'x'], coords=coords)

# works
foo.hvplot('x', 'y', label='Foo') 

[maximlt]

Thanks @jsignell ! I have added a couple of tests in c206b3b

Another note for myself :) I was wondering why label took precedence over value_label in the fix. That's actully already baked in in multiple places in the converter, so the fix nicely aligns that.

hvplot/hvplot/converter.py

Line 1164 in 447199a

name = data.name or self.label or self.value_label