oc.decode resolver failing to parse "???" as MISSING

Describe the bug

I have been trying to get some values from the envvars and I would like to have some default values in case the variable is not found. For that purpose, I've been using the missing value indicator ??? alongside oc.decode to retrieve any potential value, but the ??? is returned as a string and doesn't throw MissingMandatoryValue exception.

To Reproduce

>>> cfg = OmegaConf.create(
>>>     {
>>>         "my_key": "${oc.decode:${oc.env:MY_ENVVAR,???}}"
>>>     }
>>> )
>>> print(cfg.my_key)
???
>>> print(type(cfg.my_key)))
<class 'str'>

Expected behavior Accessing cfg.my_key should throw MissingMandatoryValue.

Additional context

[x] OmegaConf version: 2.2.3
[x] Python version: 3.9.4
[x] Operating system: Ubuntu 20.04
[x] Please provide a minimal repro

omry / omegaconf

oc.decode resolver failing to parse "???" as MISSING #1059