fix: M-07 Attempts to Bridge WETH Using ZkStack_CustomGasToken_Adapter Will Fail

[bmzig]

In order to bridge tokens from Ethereum to a ZkStack blockchain using a custom gas token, the relayTokens function of the ZkStack_CustomGasToken_Adapter contract can be used. In case the token to bridge is WETH, the token is first converted to ETH, and then that ETH is bridged using the requestL2TransactionTwoBridges function of the BridgeHub contract. The requestL2TransactionTwoBridges function then calls the bridgeHubDeposit function of the second bridge with the ETH amount specified by the caller.

However, the bridgeHubDeposit function requires that the deposit amount specified equals 0 in case when ETH is bridged, yet it is specified as a nonzero amount inside the relayTokens function of the adapter. This will cause any attempt to bridge WETH to L2 to revert.

In cases where WETH is being bridged, consider setting the amount to be used in the second bridge's calldata to 0.

This PR applies the suggestion and forces the _depositAmount field in the second bridge calldata to zero.