Can wiping a disk writing zeros on an encrypted device be a weak point?
The output of an encrypted stream of zeros is absolutely indistinguishable from random data.

What you do is not write zeros to the block device for the drive, but for the encrypted mapping. That is, instead of writing zeros to /dev/sdx, you write to /dev/mapper/sdx_crypt. This results in the contents on the actual drive being completely randomized. Even though an attacker may know that zeros were encrypted, they would be no closer to finding the key than before. If they were, it would mean the underlying cipher is vulnerable to a known plaintext attack, and none of the supported LUKS ciphers (AES, Twofish, Serpent, etc.) are. In fact, any symmetric cipher that was badly vulnerable to a known plaintext attack would be considered fatally broken, and would be either revised or discarded.

To prove a point, you can see that many high-quality cryptographic random number generators work by encrypting a counter with a block cipher. An attacker knows exactly what was encrypted by the block cipher. They know that the nth block of output is an encryption of integer n itself, yet it doesn't make it possible for them to predict any of the output or retrieve the key. Ciphers are stronger than you think.