Can zero-transactions block generation be called as a Sybil attack?
Can I call such intentional easy-mining a Sybil attack?


No. Because mining is a memory-less process, it is quite possible that subsequent blocks be found while the mempool is empty. Furthermore, the idea that verifying transactions takes away mining resources is moot since the SHA256 hashing is typically done on specialised hardware that can't do anymore than SHA256 hashing, so verification effort and mining effort is mutually exclusive.