Proto3's optional keyword causes issues in zap code generation

[stewartboyd119]

Proto3 introduced version 3.12 changelog introduced the optional field label.

However, a message like this:

message Sender { optional string polaris_id = 1; }

generated code like this

`
keyName = "polaris_id" // field polaris_id = 1
if ov, ok := m.GetXPolarisId().(*Sender_PolarisId); ok {
_ = ov
enc.AddString(keyName, ov.PolarisId)
}

`

However, the GetXPolarisId() doesn't exist and creates a compiletime error.