bEIGEN: Allow burns while transfer restrictions are enabled

[viktorking7]

Description

• Summary: Enable ERC20 burns under active transfer restrictions by permitting transfers to the zero address.
• Motivation: Public burn() was effectively blocked for non-whitelisted holders; this aligns mint/burn symmetry and expected ERC20 behavior.
• Change: In BackingEigen._beforeTokenTransfer add to == address(0) allowance and a clarifying comment.