Feat : apply optional is nullable

[xeromank]

summary

• Optional fields are nullable.

story

• Hey, I'm back again. First of all, thank you so much, I'm still using your open source.
• Today's request is...
• Currently, in the Swagger schema created through this, the optional fields are expressed as follows.
• But our front engineer requested that it be expressed as below.
• It was explained well in link here.
• So, I modified the code a little to express it as above, relying on the optional field to handle the nullable of jsonSchema.

Thanks

Please take care of this Pull Request as well. Finally, I'm not proficient in English, so I wrote this text using a translation tool. Please keep that in mind. Thank you!!

[xeromank]

I didn't check the test properly, sorry. I'll fix it quickly.

[xeromank]

I handled it for 'oneof'. thank you!! Thanks!!