Projection Extension migrate doesn't handle "proj:epsg": None case correctly

[Kirill888]

When "proj:epsg": None is set on input, migrate code will generate "proj:code": "EPSG:None" on output breaking CRS access.

Culprit is here:

pystac/pystac/extensions/projection.py

Lines 480 to 483 in 4dc0e0f

	if "proj:epsg" in obj["properties"]:
	epsg = obj["properties"]["proj:epsg"]
	obj["properties"]["proj:code"] = f"EPSG:{epsg}"
	del obj["properties"]["proj:epsg"]

it should include check for None in there.