The g'mara in B'rachos (4B) explains that 'נ' represents downfall [of the nation] and is therefore encompassed in the positive context of the putative next pasuk, which states that "God supports all of the fallen".
See this post on On The Main Line about a nun verse in Ashrei from antiquity, and whether is was original:
As it turns out, at Qumran a Hebrew version of tehillim, Psalms, was found (11QPs-a) which contains a nun verse--in Hebrew--a version pretty close, but not identical, with the Septuagint verse. In fact, it read ne'eman adonay be-khol derakhav ve-hasid be-khol ma'asav, which is to say, the exact same as the tsade verse, tsadik adonay be-khol derakhav ve-hasid be-khol ma'asav, except for the first word. This version is close to the LXX, excepting that the LXX reads "holy" for hasid, "gracious," in the Qumran text--which, by the way, isn't a Psalm scroll per se, but appears to be a text for liturgical use.
Thus, it is clear that there was a Hebrew version of this psalm with a nun verse in antiquity.
The question is, is it original?