define Default zero address

[sirano11]

Proposed changes

This PR changes the default value of the address from 0x to 0x0000000000000000000000000000000000000000 in 20 bytes. Since Klaytn's address has a fixed length of 20 bytes, it cannot have 0x as a default.

• Define default zero address in caver.utils. : DEFAULT_ZERO_ADDRESS
• from, fee payer field in Transaction set default value as 0x0000000000000000000000000000000000000000

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[jimni1222]

LGTM :) Thank you for changing!