Value is None when set to optional env var and not overridden in included conf with same structure

[VolodymyrFrolov]

To reproduce the issue several things need to come together:

1. Value on 2nd+ level of nestiness.
2. Value should be set via optional env var syntax ${?FOO}.
3. There should be another config referenced with same structure but w/o overriding that value.
4. Env var must be unset.

Please see an example for above (url2 added for use-case example only and does not affect reproducibility):

app.conf:

root {
  level1 {
    url1 = "http://default-1"
    url1 = ${?FOO}
    url2 = "http://default-2"
  }
}

include "dev.conf"

dev.conf:

root {
  level1 {
    url2 = "http://dev-2"
  }
}

test.py:

from pyhocon import ConfigFactory
print(ConfigFactory.parse_file('app.conf').get_string('root.level1.url1'))

Expected: http://default-1
Actual: None

Reproduced with v0.3.61, Python 3.9