Hi JIm,
In any operating system, there is always a default profile at work there, so I don’t think you would ever have a situation where there really was no profile Photoshop could use. Whenever there are problems, like if PS does not like the current monitor profile (it’s corrupted somehow) - PS will give you a little warning dialog and announce that it is substituting sRGB.
In a display profile, it is specifically the vctg tag that is grabbed by the graphics card to get the info for the LUTs.