Media type argument in `Item.get_links` method is ignored

[pjhartzell]

The media_type argument is ignored when filtering the returned links with the Item.get_links method. It looks like it is short-circuited by the absence of a rel argument.

pystac/pystac/stac_object.py

Lines 125 to 133 in e77cd0a

	if rel is None:
	return self.links
	else:
	return [
	link
	for link in self.links
	if link.rel == rel
	and (media_type is None or link.media_type == media_type)
	]

I would expect to be able to filter on either (or both) rel and media_type. For example, I would expect the following to return the single link with an HTML media type, but it returns all links.

import pystac

item = pystac.read_file("https://planetarycomputer.microsoft.com/api/stac/v1/collections/landsat-c2-l2/items/LC09_L2SR_083122_20221230_02_T2")

html_links = item.get_links(media_type=pystac.MediaType.HTML)
for link in html_links:
    print(link.to_dict())

# Output:
# {'rel': 'collection', 'href': 'https://planetarycomputer.microsoft.com/api/stac/v1/collections/landsat-c2-l2', 'type': 'application/json'}
# {'rel': 'parent', 'href': 'https://planetarycomputer.microsoft.com/api/stac/v1/collections/landsat-c2-l2', 'type': 'application/json'}
# {'rel': <RelType.ROOT: 'root'>, 'href': 'https://planetarycomputer.microsoft.com/api/stac/v1/', 'type': <MediaType.JSON: 'application/json'>, 'title': 'Microsoft Planetary Computer STAC API'}
# {'rel': 'self', 'href': 'https://planetarycomputer.microsoft.com/api/stac/v1/collections/landsat-c2-l2/items/LC09_L2SR_083122_20221230_02_T2', 'type': 'application/geo+json'}
# {'rel': 'cite-as', 'href': 'https://doi.org/10.5066/P9OGBGM6', 'title': 'Landsat 8-9 OLI/TIRS Collection 2 Level-2'}
# {'rel': 'via', 'href': 'https://landsatlook.usgs.gov/stac-server/collections/landsat-c2l2-sr/items/LC09_L2SR_083122_20221230_20230101_02_T2_SR', 'type': 'application/json', 'title': 'USGS STAC Item'}
# {'rel': 'preview', 'href': 'https://planetarycomputer.microsoft.com/api/data/v1/item/map?collection=landsat-c2-l2&item=LC09_L2SR_083122_20221230_02_T2', 'type': 'text/html', 'title': 'Map of item'}