Can't assign None to optional field currently interpolated to a non-optional one

[odelalleau]

Describe the bug

The following code should work, but currently crashes with ValidationError: child 'b' is not Optional:

@dataclass
class Config:
    a: int = 3
    b: Optional[int] = "${a}"
        
cfg = OmegaConf.structured(Config)
cfg.b = None

To Reproduce
See above

Expected behavior
No error

Additional context
This is on current master (8103a84)