dingtalk _extra_get, mattermost _extra_or_env and slack _extra_or_env_flag/_channel_set fell
through to env only on None, so `allowed_channels: ""` / `free_response_channels: ""` meant
"no whitelist" rather than "use the env CSV". The shared reader treated blank as unset and
silently widened those to the env value. New `blank_is_unset=False` knob restores the old
semantics at those seven call sites; the default (blank = unset) stays for the readers whose
old body was `extra.get(k) or env`.
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