fix: handling of PROTOCOL_HEADER and HOST_HEADER env vars

[Conduitry]

Fixes #14218, hopefully. This simply reverts the change from ??s to ||s. The values here being empty strings might indicate the environment variable not being set (in which case we definitely want to fall back to the default values) or it might indicate the env var being set and the header being set but to an empty value (in which case, it would be more helpful to also fall back to the default values). Using ?? over || is generally a worthwhile endeavor for more strict correctness, but not in this case.

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