MissingFieldException, however field is present

[Him188]

mamoe/mirai-console#191

[Him188]

Simplified reproducer:

@Serializable
data class User(
    val mailAddress: String? = null,
    val name: String,
)

default.decodeFromString(User.serializer(), """
name: cssxsh
mailAddress: [e-mail]
""")
    }

[Him188]

Root cause is wrong decodeNullableSerializableElement