Generate mipmaps at runtime if levelCount=0

𝑙𝑒𝑣𝑒𝑙𝐶𝑜𝑢𝑛𝑡=0 is allowed, except for block-compressed formats, and means that a file contains only the base level and consumers, particularly loaders, should generate other levels if needed.

KTX specification, Section 3.7

So according to this, KtxUnity should ensure that mipmaps are generated for those types of ktx files.