Support Zero value time.Time for date and date32

[KevinJoiner]

Describe the bug

zero value for a time.Time object is inconsistently handled between the Date and DateTime types.
If a user uses time.Time{} for a DateTime or a DateTime32 object, it gets translated to UNIX time of 0. But this is not the case for the ToDate functions for Date and Date32. Currently if time.Time{} is used for Date object the resulting date is 1974-10-01 due to casting a negative unix time to a uint.

Steps to reproduce

1. Create a zero-valued time
2. convert it to a Date and a DateTime
3. Notice inconsistent Dates

Expected behaviour

Both Date and date time convert time.Time{} to the same value, preferably 0

Code example

package main

import (
	"fmt"
	"time"

	"github.com/ClickHouse/ch-go/proto"
)

func main() {
	zeroTime := time.Time{}
	date := proto.ToDate(zeroTime)
	dateTime := proto.ToDateTime(zeroTime)
	fmt.Printf("Date: %d\n", date)
	fmt.Printf("DateTime: %d\n", dateTime)
	// Output: Date: 1734
	// 		   DateTime: 0
}