Fix fmt.Stringer.String() called on nil pointer

[wilhelmliao]

Summary

got error when call bind() using parameter (*fmt.Stringer)(nil).

var v *json.Number  // fmt.Stringer
query := "SELECT ?"
params := []interface{}{
v,
}
sql, err := bindPositional(time.UTC, query, params...)
fmt.Println(sql)

want

SELECT NULL

got

panic: value method encoding/json.Number.String called using nil *Number pointer

Checklist

Delete items not relevant to your PR:

• [v ] Unit and integration tests covering the common scenarios were added

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[jkaflik]

@wilhelmliao could you remove conflicts?

[wilhelmliao]

I fixed the conflicts, but the patched codes seem missing.

https://github.com/wilhelmliao/clickhouse-go/blob/main/bind.go#L308:313

	case fmt.Stringer:
		rv := reflect.ValueOf(v)
		if rv.Kind() == reflect.Pointer {
			if rv.IsNil() {
				return "NULL", nil
			}
		}
		return quote(v.String()), nil

[jkaflik]

@wilhelmliao yes, there was a PR improving bind - #1092.

Thanks for your effort!