[PHP] Add support to nullable (based on #3493)

[thomasphansen]

This is basically PR #3493, rebased, adjusted to the latest code and with some bug fixes and code modernization. @githubERIK, the author of the original PR, said he doesn't plan to continue with that (#3493 (comment)), but we have been using his code in our fork for more than 2 years, with no issues! 😄

His biggest concert was support for openapi 2.0. But this is not a problem at all: OpenAPIGenerator is setting isNullable to true if 'x-nullable' is defined in the swagger file, which means, this code works OOTB with openapi 2.0, provided that x-nullable is properly set.

The most important changes to the original code are:

• model_generic.mustache, method isNullable(): changed to use self::openAPINullables() method instead of the property itself. This is a bugfix: if we don't do this, parent properties will not be checked.
• model_generic.mustache, methods getOpenAPINullablesSetToNull() and setIfExists(): changed to private, since they are internal to the model class.
• model_generic.mustache, method openAPINullables(): changed to protected, since it is only used within class and parent scope.
• model_generic.mustache, method setIfExists(): changed to return void, as it is used only internally and its return is never used.
• included isNullable() and isNullableSetToNull() to ModelInterface.mustache, which didn't exist when the original PR was presented
• modernized PR code (null coalesce operator, return types, missing types for properties)

If the original author prefers, I can present these changes as a patch to his own PR, or he can add me to his branch so that I can merge them myself. The only important thing for me is to see the feature in OpenAPIGenerator base code, as a) it works very well! 😄 and b) this would help us moving away from using a fork 😉

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[wing328]

PHP tests passed via https://app.travis-ci.com/github/OpenAPITools/openapi-generator/builds/253054605

[wing328]

@thomasphansen thanks for the PR. Let's move ahead with this change.