[Bug]: Golang SDK doesn't work with bigquery.NullDate

### What happened?

Using Go SDK, I'd like to create a job to import avro data and then export to bigquery.

My script is like:

```go
package main

import (
"context"
"encoding/json"
"flag"
"fmt"
"log"
"reflect"
"time"

"cloud.google.com/go/bigquery"
"cloud.google.com/go/civil"
"github.com/apache/beam/sdks/v2/go/pkg/beam"
"github.com/apache/beam/sdks/v2/go/pkg/beam/io/avroio"
"github.com/apache/beam/sdks/v2/go/pkg/beam/io/bigqueryio"
"github.com/apache/beam/sdks/v2/go/pkg/beam/x/beamx"
)

var (
projectID = flag.String("project_id", "my_project_id", "GCP project ID")
input = flag.String("input", "gs://my_bucket/hoge.avro", "Input avro file")
dataset = flag.String("dataset", "test_dataset", "Output Bigquery dataset")
table = flag.String("table", "test_table", "Output Bigquery table")
)

type Sample struct {
ID bigquery.NullInt64 `bigquery:"id" json:"id"`
Date bigquery.NullDate `bigquery:"date" json:"date"`
}

func AppendDate(src string) Sample {
var s Sample
_ = json.Unmarshal([]byte(src), &s)
s.Date = bigquery.NullDate{Valid: true, Date: civil.DateOf(time.Now())}
return s
}

func main() {
flag.Parse()

beam.Init()
p := beam.NewPipeline()
s := p.Root()

orig := avroio.Read(s, *input, reflect.TypeOf(""))

format := beam.ParDo(s, AppendDate, orig)

t := fmt.Sprintf("%s:%s.%s", *projectID, *dataset, *table)
bigqueryio.Write(s, *projectID, t, format)

ctx := context.Background()
if err := beamx.Run(ctx, p); err != nil {
log.Fatal(err)
}
}
```

Unfortunately I got error below **only when the runner is `dataflow`**

```
$ go run cmd/terminal_info/main.go --runner dataflow

2022/06/28 16:16:22 No environment config specified. Using default config: 'apache/beam_go_sdk:2.39.0'
2022/06/28 16:16:22 failed to convert type main.Sample to a schema.
Full error:
generating model pipeline
failed to add input kind: {main.AppendDate 5: ParDo [In(Main): string <- {4: string/string GLO}] -> [Out: main.Sample -> {5: main.Sample/R[main.Sample] GLO}]}
caused by:
cannot convert field Date to schema
caused by:
unable to convert bigquery.NullDate to schema field
caused by:
cannot convert field Date to schema
caused by:
unable to convert civil.Date to schema field
caused by:
cannot convert field Month to schema
caused by:
unable to map time.Month to pipepb.AtomicType
exit status 1
```

According to the message, type in `bigquery.NullDate` seems not acceptable for this sdk.

I understand this can be avoided by primitive type like

Date int64  `bigquery:"date" json:"date"`

However I want to set `NullDate` literally.

Do you have any ideas to solve this error?

And then, the weird thing is I don't get the above error **when I use `DirectRunner`**. If you have ideas about this too, I'd like to know.

```
go run cmd/terminal_info/main.go
2022/06/28 16:26:07 Executing pipeline with the direct runner.
2022/06/28 16:26:07 Pipeline:
2022/06/28 16:26:07 Nodes: {1: []uint8/bytes GLO}
{2: string/string GLO}
{3: string/string GLO}
{4: string/string GLO}
{5: main.Sample/R[main.Sample] GLO}
{6: KV/KV GLO}
{7: CoGBK/CoGBK GLO}
Edges: 1: Impulse [] -> [Out: []uint8 -> {1: []uint8/bytes GLO}]
2: ParDo [In(Main): []uint8 <- {1: []uint8/bytes GLO}] -> [Out: T -> {2: string/string GLO}]
3: ParDo [In(Main): string <- {2: string/string GLO}] -> [Out: string -> {3: string/string GLO}]
4: ParDo [In(Main): string <- {3: string/string GLO}] -> [Out: X -> {4: string/string GLO}]
5: ParDo [In(Main): string <- {4: string/string GLO}] -> [Out: main.Sample -> {5: main.Sample/R[main.Sample] GLO}]
6: ParDo [In(Main): T <- {5: main.Sample/R[main.Sample] GLO}] -> [Out: KV -> {6: KV/KV GLO}]
7: CoGBK [In(Main): KV <- {6: KV/KV GLO}] -> [Out: CoGBK -> {7: CoGBK/CoGBK GLO}]
8: ParDo [In(Main): CoGBK <- {7: CoGBK/CoGBK GLO}] -> []
2022/06/28 16:26:07 Plan[plan]:
9: Impulse[0]
1: ParDo[bigqueryio.writeFn] Out:[]
2: CoGBK. Out:1
3: Inject[0]. Out:2
4: ParDo[beam.addFixedKeyFn] Out:[3]
5: ParDo[main.AppendDate] Out:[4]
6: ParDo[avroio.avroReadFn] Out:[5]
7: ParDo[avroio.expandFn] Out:[6]
8: ParDo[beam.createFn] Out:[7]
...
```

Thank you.

### Issue Priority

Priority: 1

### Issue Component

Component: sdk-go