Omit zero time.Time default from usage line

[mologie]

This was missed in #348, there previously was no way to omit the default value. Treating zero timestamp values as canary for absence of a default is in line with other flag types, e.g. zero ints.

Stand-alone test program:

package main

import (
	"github.com/spf13/pflag"
	"time"
)

func main() {
	pflag.Time("time", time.Time{}, []string{time.RFC3339Nano, time.RFC3339}, "test")
	pflag.Parse()
	pflag.Usage()
}

Before this change:

Usage of pflagtest:
      --time time   test (default 0001-01-01T00:00:00Z)

After:

Usage of pflagtest:
      --time time   test

[CLAassistant]

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