Behavior change: Merging a MISSING node should not change target

[omry]

$ cat 1.py 
from omegaconf import OmegaConf

cfg1 = OmegaConf.create({"a" : 1, "b" : "???"})
cfg2 = OmegaConf.create({"a" : "???", "b" : 2})
print(OmegaConf.merge(cfg1, cfg2))

 
omry@Coronadev:~/dev/omegaconf$ python 1.py 
{'a': '???', 'b': 2}

Desired behavior is for missing fields being merged onto non-missing fields to not change them.
Desired output:

{'a': 1, 'b': 2}

[omry]

cc @odelalleau