Update allOf to not always be nullable

This is part of a larger PR that is being broken up. Updates allOf to not always be nullable.

To validate this change, I will focus on AllOfWithSingleRef.cs. Before the change, I edited modelGeneric.mustache to determine what properties were determining the EmitDefaultValue change by adding a /comment/ after the boolean.

...EmitDefaultValue={{#vendorExtensions.x-emit-default-value}}true /*venderExtension*/{{/vendorExtensions.x-emit-default-value}}{{^vendorExtensions.x-emit-default-value}}{{#isNullable}}true /*nullable*/{{/isNullable}}{{^isNullable}}{{#required}}true /*required*/{{/required}}{{^required}}false /*not required*/{{/required}}...

The result is that EmitDefaultValue is being determined by the property always being nullable:

After the change, we see

So we see the change makes the allOf neither required nor nullable. The input spec for this sample is modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml. Below is the definition for this object. It is not nullable in the spec, and it is no longer nullable in code.

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