Passing null values to queries, Sprout Reports

In Sprout Reports, I am creating a list of users in my settings file. I first need an option with value of null.

{% set options = [] %}
{% set options = options|merge([{ label: 'All users', value: null }]) %}
{% for user in users %}
  {% set fullname = user.firstName ~ " " ~ user.lastName %}
    {% set options = options|merge([{ label: fullname, value: user.id }]) %}
{% endfor %}

{{ forms.selectField({
    label: "Choose one"|t,
    name: "userId",
    options: options,
    }) 
}}

Then, in my index file (the report), I do something like this:

{% set purchases = craft.entries()
  .section('purchases')
  .authorId(settings.userId)
  .all()
%}

My query runs fine if there is a value for userId, but when it is null, the query does not return any results.

What am I doing wrong?

Do I need to do something like this?

{# Start your element query #}
{% set query = craft
    .entries()
    .section('purchases')
%}

{# Check if settings.userId, if so, apply it to the query #}
{% if settings.userId %}
    {% set query = query.authorId(settings.userId) %}
{% endif %}

{# Run the query #}
{% set purchases = query.all() %}