Added "skip nil" (?) prefix flag

[joente]

Description

To reduce size, it can be useful to not return nil values. For example, support type T has a property with definition str? and this property is nil most of the time, you might save bandwidth to not include the value when nil.

This pull request adds the ? prefix flag which tells to exclude from a result when nil.

Example:

set_type('A', {
    name: 'str',
    description: 'str?',  // will return when nil
});
set_type('B', {
    name: 'str',
    description: '?str?',  // do not return when nil
});

[A{name: 'a'}.wrap(), B{name: 'a'}.wrap()];

Returns:

[
    {
        "description": null,
        "name": "a"
    },
    {
        "name": "a"
    }
]

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Test wrap contains the test code

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes