breaking change in 0.3.60

[carolguo-dd]

Seems like 0.3.60 has introduced another problem.

{
     a = 3
    test = ${a}
    test = ${?b}
    result = ${test}
}

this used to work in 0.3.59 version, but now it breaks with

  File "/Users/carol.guo/.pyenv/versions/3.9.7/lib/python3.9/site-packages/pyhocon/config_parser.py", line 695, in resolve_substitutions
    raise ConfigSubstitutionException("Cannot resolve {variables}. Check for cycles.".format(
pyhocon.exceptions.ConfigSubstitutionException: Cannot resolve ${test}: (line: 7, col: 21). Check for cycles.

[carolguo-dd]

created a pr in attempt to fix the problem

[DannyVarod]

@carolguo-dd does this PR also solve this?: https://github.com/chimpler/pyhocon/issues/300