As of version 3.7.3, Zanata does not allow to add a comment for an entry without translation.
There is a case where a translator wants to leave a comment even when an entry has no translation.
For example, for a case where an original string is difficult to translation, a translator wants to leave a comment about why it is not/cannot be translated now, or a comment that this is being fixed in an upstream project (see bug XXXX).
I believe it is useful to allow a translator to add a comment even when an entry has no translation.
I am using version 3.7.3 (Zanata OpenStack instance).