Avoid calling `getSerializableData()`

[cebe]

moved from cebe#123

cebe#112 (comment)

Calling getSerializableData() is a significant performance overhead which is currently necessary because of how php-openapi handles default values. We should try to avoid that.

[SOHELAHMED7]

In order to fix this issue

The method getSerializableData() is used in resolving property and faker. For resolving properties, I am changing the implementation as per spec: default value of nullable is false.

As per current implementation

• default value of nullable in true which is incorrect as per https://github.com/OAI/OpenAPI-Specification/blob/main/proposals/2019-10-31-Clarify-Nullable.md
• if nullable is false, we set isRequired to true which is also incorrect. Example: a column with data type string and nullable false can have non-null empty string value

Pull request #98 will

• fix this incorrect behaviour
• make this lib more OpenAPI spec compliant
• get rid of getSerializableData() in resolving properties