Fix gen code without chrono to handle nullable logical-dates

[kebbestrom]

I use this tool to generate my Rust code from Avro schemas, but I want to use something other than Chrono dates. Instead, I would use the i64 version for my logical dates.

The code currently being generated for nullable dates needs to handle this correctly. It will incorrectly try serializing the Option<i64> using chrono::serde::ts_milliseconds_option.

Here is an example of generating the Avro schema logical_dates.avsc with chrono dates disabled:

/// Date type
#[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize, serde::Serialize)]
pub struct DateLogicalType {
    pub birthday: i32,
    #[serde(with = "chrono::serde::ts_milliseconds_option")]
    #[serde(default = "default_datelogicaltype_meeting_time")]
    pub meeting_time: Option<i64>,
    #[serde(default = "default_datelogicaltype_release_datetime_micro")]
    pub release_datetime_micro: i64,
}

#[inline(always)]
fn default_datelogicaltype_meeting_time() -> Option<i64> { None }

#[inline(always)]
fn default_datelogicaltype_release_datetime_micro() -> i64 { 1570903062000000 }

[lerouxrgd]

Thanks for the fix !