Handle tokens that prevent zero transfer

[EndymionJkb]

Per auditor comment (and we know this from v1/v2), some tokens can't handle zero value transfers. While most of the time this is checked (often as a gas optimization), there are places where it's not. We don't need to do it for WETH or BPT, but any time there is an unknown "pool token," we can ensure transfers are non-zero.

Hopefully it will be mostly a wash as far as gas. The checks take some time, but we're also avoiding zero transfers.