yaml marshalling with omitempty is broken due to IsZero on pointer

[prashantv]

The switch to pointer receivers to fix #106 broke YAML marshalling of strfmt.DateTime when used with a omitempty value field.

Repro:

``` package main

import (
"fmt"
"time"

    "github.com/go-openapi/strfmt"
    "gopkg.in/yaml.v3"

)

type S struct {
Name string yaml:"name"
Created strfmt.DateTime yaml:"created,omitempty"
}

func main() {
s := S{
Name: "test",
Created: strfmt.DateTime(time.Now()),
}
marshalled, err := yaml.Marshal(s)
if err != nil {
panic(err)
}

    fmt.Println(string(marshalled))

}

</details>

With 0.21.5, the output of the above is:

name: test
created: "2025-01-09T16:13:43.220-08:00"

With 0.21.6, and all newer versions including 0.23.0, the output is:

name: test

(no output for `created`).

The primary reason is that when the field is specified as a value, only value methods can be called.

The fix was originally added to because `swag.IsZero` did not handle pointer, but that was fixed in https://github.com/go-openapi/swag/pull/67 so I think the the `IsZero` method should be moved back to the value.