Change Option<T> compose to OneOfBuilder

This commit change the `Option<T>` compose implementation from
`AllOfBuilder` to `OneOfBuilder`. This comes as follow up for #1135
which changed the `utoipa-gen` side nullable types from `allOf` to
`oneOf`.

Closes #1050 Closes #1135

utoipa/src/lib.rs

@@ -1360,7 +1360,7 @@ pub mod __dev {
         fn compose(
             schemas: Vec<utoipa::openapi::RefOr<utoipa::openapi::schema::Schema>>,
         ) -> utoipa::openapi::RefOr<utoipa::openapi::schema::Schema> {
-            utoipa::openapi::schema::AllOfBuilder::new()
+            utoipa::openapi::schema::OneOfBuilder::new()
                 .item(
                     utoipa::openapi::schema::ObjectBuilder::new()
                         .schema_type(utoipa::openapi::schema::Type::Null),