[REQ][cppqt] Support nullable integer fields

[martonmiklos]

I need to work with an API (InvenTree) which specifies some fields as a nullable integer.

parent:
          type: integer
          nullable: true

This is modeling a relation where the null have a special meaning: no relation, and it is not identical to specifying 0.
Unfortunately with the current generated API I am unable to specify the fields as null.

Describe the solution you'd like

For these fields the openapi-codegen generates the following API members ():

    qint32 getParent() const;
    void setParent(const qint32 &parent);
    bool is_parent_Set() const;
    bool is_parent_Valid() const;

I would extend it with the following:

    qint32 getParent() const;
    bool isParentNull() const;
    void setParent(const qint32 &parent);
    void setParentNull();
    bool is_parent_Set() const;
    bool is_parent_Valid() const;

@etherealjoy: I seen you were the main author of the Qt stuff, I would glad to hear you opinion on this.