nil pointer field cause panic

[m1heng]

Description

After add otellogrus hook, when logging with a field which contains a nil pointer would cause internal panic

Environment

• OS: any
• Architecture: any
• Go Version: 1.23.0
• go.opentelemetry.io/contrib/bridges/otellogrus version: 0.5.0

Steps To Reproduce

package main

import (
	"github.com/sirupsen/logrus"
	"go.opentelemetry.io/contrib/bridges/otellogrus"
)

type ATopicHandler struct {
	some string
}

func main() {
	var a *ATopicHandler
	logger := logrus.New()
	hook := otellogrus.NewHook("test")
	logger.AddHook(hook)
	logger.WithField("a", a).Error("error")
}

panic: reflect: call of reflect.Value.Interface on zero Value

goroutine 1 [running]:
reflect.valueInterface({0x0?, 0x0?, 0x0?}, 0x0?)
        /Users/m1heng/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.0.darwin-arm64/src/reflect/value.go:1486 +0xfc
reflect.Value.Interface(...)
        /Users/m1heng/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.0.darwin-arm64/src/reflect/value.go:1481
go.opentelemetry.io/contrib/bridges/otellogrus.convertValue({0x10446e300, 0x0})
        /Users/m1heng/go/pkg/mod/go.opentelemetry.io/contrib/bridges/otellogrus@v0.5.0/hook.go:233 +0x3e4
go.opentelemetry.io/contrib/bridges/otellogrus.convertFields(0x1400011ca80)
        /Users/m1heng/go/pkg/mod/go.opentelemetry.io/contrib/bridges/otellogrus@v0.5.0/hook.go:180 +0xec
go.opentelemetry.io/contrib/bridges/otellogrus.(*Hook).convertEntry(_, _)
        /Users/m1heng/go/pkg/mod/go.opentelemetry.io/contrib/bridges/otellogrus@v0.5.0/hook.go:170 +0x9c
go.opentelemetry.io/contrib/bridges/otellogrus.(*Hook).Fire(0x1400011c9c0, 0x1400010ad08?)
        /Users/m1heng/go/pkg/mod/go.opentelemetry.io/contrib/bridges/otellogrus@v0.5.0/hook.go:159 +0x38
github.com/sirupsen/logrus.LevelHooks.Fire(0x1400010ad48?, 0x10ad08?, 0x1400015c0e0)
        /Users/m1heng/go/pkg/mod/github.com/sirupsen/logrus@v1.9.3/hooks.go:28 +0x78
github.com/sirupsen/logrus.(*Entry).fireHooks(0x1400015c0e0)
        /Users/m1heng/go/pkg/mod/github.com/sirupsen/logrus@v1.9.3/entry.go:280 +0x20c
github.com/sirupsen/logrus.(*Entry).log(0x1400015c070, 0x2, {0x140001020fa, 0x5})
        /Users/m1heng/go/pkg/mod/github.com/sirupsen/logrus@v1.9.3/entry.go:242 +0x3b0
github.com/sirupsen/logrus.(*Entry).Log(0x1400015c070, 0x2, {0x1400010af28?, 0x10446e300?, 0x0?})
        /Users/m1heng/go/pkg/mod/github.com/sirupsen/logrus@v1.9.3/entry.go:304 +0x60
github.com/sirupsen/logrus.(*Entry).Error(...)
        /Users/m1heng/go/pkg/mod/github.com/sirupsen/logrus@v1.9.3/entry.go:333
main.main()
        /Users/m1heng/xxxxx/main.go:19 +0x134
exit status 2

Expected behavior

log normally