[BUG] ImageBuf::nmiplevels() returns 0

[AlexeySmolenchuk]

OpenImageIO 3.1.14.0

Only cached version of ImageBuf object returns correct number of mipmaps.

buffer = ImageBuf(imageFileName, subimage, mip, cache);
nmips = buffer.nmiplevels(); // Works as expected

buffer = ImageBuf(imageFileName, subimage, mip);
nmips = buffer.nmiplevels(); // Always returns zero

Same for both exr and tiff backends.

Could be the same issue: #4984