Calling .only() on a field that has not been set does not go to default value

I've had this issue a few times now, where I'm calling .only() for efficiency, and then when using the value, I don't get the default, I get None returned.

Other fields not specified in the .only() function do get set to their default value, however.

Reproduce:

class Example(Document):
    meta = {"collection": "example"}

    str1 = StringField(default="hi")
    str2 = StringField(default="hi2")

Example().save()
a = Example.objects.only("str1").first() # a.str1 == None, a.str2 == "hi2"

Start with the provided Example document and the Example.objects.only("str1").first() reproduction, then trace how .only() loads unspecified fields and applies defaults. Done means the reproduced str1 access returns "hi" while the projection behavior remains intact, with a regression test covering the example.