catch `image=None` in `_retryer()`

[veenstrajelmer]

Follow-up from #247

There are still Nonetype related errors after fixing #248.

When array[0] is None, the code fails on line 682 with an AttributeError:

/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/site-packages/contextily/plotting.py:134: in add_basemap
    image, extent = bounds2img(
/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/site-packages/contextily/tile.py:287: in bounds2img
    merged, extent = _merge_tiles(tiles, arrays)
/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/site-packages/contextily/tile.py:682: in _merge_tiles
    h, w, d = arrays[0].shape
E   AttributeError: 'NoneType' object has no attribute 'shape'

If the first array is valid, but any of the other arrays is None, the issue surfaces a bit lower as a TypeError:

/opt/hostedtoolcache/Python/3.12.6/x64/lib/python3.12/site-packages/contextily/plotting.py:134: in add_basemap
    image, extent = bounds2img(
/opt/hostedtoolcache/Python/3.12.6/x64/lib/python3.12/site-packages/contextily/tile.py:287: in bounds2img
    merged, extent = _merge_tiles(tiles, arrays)
/opt/hostedtoolcache/Python/3.12.6/x64/lib/python3.12/site-packages/contextily/tile.py:692: in _merge_tiles
    img[y * h : (y + 1) * h, x * w : (x + 1) * w, :] = arr
E   TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

I cannot fully grasp what is happening here. I imagined that image=None, and was looking at this part of the code:

contextily/contextily/tile.py

Lines 454 to 457 in 0e38541

	with io.BytesIO(request.content) as image_stream:
	image = Image.open(image_stream).convert("RGBA")
	array = np.asarray(image)
	image.close()

So I guess that would lead to this minimal example:

import numpy as np
image = None
array = np.asarray(image)
print(array.shape)

However, this prints () and does not give the error above. @martinfleis do you have an idea how the image variable looks like when this error occurs? Or is my minimal reproduction example faulty?