比如 mapping(address => uint) tester，只需要判断 mapping 是否为默认值 0， tester[msg.sender] == 0

"You can think of mappings as hash tables, which are virtually initialised such that every possible key exists and is mapped to a value whose byte-representation is all zeros, a type’s default value."

Docs：https://solidity.readthedocs.io/en/v0.6.3/types.html#mapping-types

Link：https://www.cnblogs.com/farwish/p/12362647.html

[Contract] Solidity 判断 mapping 值的存在