Fix parse exception when TimeField initial value is not set

[chikamura]

When the initial value of TimeField is not set, the get fields api returns the default value as an empty string.
This cause TimeField parse exception in getFormFields. This PR fixes that.

Text '' could not be parsed at index 0 (through reference chain: com.kintone.client.api.app.GetFormFieldsResponseBody["properties"]->java.util.LinkedHashMap["TIME"]->com.kintone.client.model.app.field.TimeFieldProperty["defaultValue"])
	at 
...

> curl -s -XGET "https://${HOST}/k/v1/app/form/fields.json?app=${APP_ID}" -H "X-Cybozu-Authorization: ${KINTONE_CRED}" | jq .properties.TIME.defaultValue
""

[chikamura]

@tomohiro-okuyama Could you please review it?

[tomohiro-okuyama]

@chikamura
Thank you for your report!
I think the fix is sufficient, so we'd like to merge this PR soon.

[chikamura]

Many thanks! 🙏