I am going to caveat my answer by first saying the Judiciary Branch of the Government has some significant flexibilites in their procurement regulations than most Executive Branch agencies.  Our expertise on this forum is Executive agencies (mostly DoD).

The answer to your primary question of "If contractors are working with no funding on the contract, is that always an ADA violation?" (bold and italics added for emphasis).  In a word; no, it is not.  The Federal Acquisition Regulation availability of funds clauses (52.232-18 and 52.232-19) put the contractor on notice that the government incurs no legal liability...  However, government officials are not authorized/allowed to "encourage" the contractor to continue working.

You were wise and we applaud you for actually reading the statute! This is another resource we have for you regarding ADA, I'll trust you will find the content and additional resources referenced and linked in this article very useful: Acquipedia ADA Article.  (https://www.dau.edu//acquipedia/pages/articledetails.aspx#!8)