It would appear1 that there is no single word that encompasses the meanings of both "lacking in justification" and "true". Therefore, a phrase must be used, and I would suggest "unsubstantiated" as the most appropriate word for the first part. Thus you could say that "Jackie Chan is a martial artist" is an unsubstantiated truth (or ...fact or ...belief).
This is, in my opinion, a better phrase than unjustified truth/belief/fact since I believe that latter carries connotations of being wrong in asserting the truth (or holding the belief etc.). Unsubstantiated just says we haven't shown it to be true or false.
The OED's entry for unsubstantiated links to the second sense of "un-":
- Prefixed to past participles, forming adjectives expressing the fact that the specified action has not been carried out.
and its entry for substantiated:
That has been substantiated (in various senses of the verb); given substance, made real. Now usually: (of a fact, claim, etc.) verified, justified with evidence.
(Emphasis mine in both cases). So an unsubstantiated fact or opinion is one where verification or justification has not happened. It does not imply that either cannot happen.
On the other hand, the OED's two most appropriate entries for unjustified are:
a. Of a person, group, etc.: lacking or unable to demonstrate good justification for an action, opinion, etc.; not vindicated or shown to be in the right; not exonerated from a charge, criticism, etc. Cf. justify v. 2a, 7a.
b. Of an action, opinion, etc.: not clearly right, proper, or appropriate; lacking justification or good cause; unwarranted.
While these definitions do not explicitly state that verification/proof is not possible, the tone is far less neutral than for unsubstantiated. In addition, the etymology of unjustified points to the first sense of "un-":
- Prefixed to adjectives to express a negative sense, forming adjectives (and derived nouns).
Together, these all point to unsubstantiated being the better word to express the lack of evidence (one way or another), whereas unjustified has strong connotations of their actually being evidence to the contrary.
1 Based on no one having found evidence so far for such a word. Of course, lack of proof is not proof of a lack.