Expected behaviour of render when passing a nil-valued parameter?

[JacobBeaudry]

I was wondering what the expected behaviour is, if one passes a nil value to a parameter, in render? e.g. if file123 is:

{% assign foo = foo | default: 'hello world' %}
This is foo in file123: {{ foo }}

and one then renders file123, passing foo as a nil-valued parameter, e.g. in parent_file1:

{% render "file123", foo: nil %}

Should it return

This is foo in file123: hello world

or

This is foo in file123:

?

And should anything change if, instead, we had, e.g. parent_file2 (in which the parameter foo is assigned to the value of the variable foo in parent_file2, in which it is not assigned (&, IINM, therefore defaults to nil)?

{% comment %} foo is not assigned anywhere in parent_file2 {% endcomment %}
{% render "file123", foo:foo %}

Hope this is clear. Thank you in advance for your time and help!

[harttle]

Scopes are inherited through include and layout calls, can be visualized as calling tree like:

entry.liquid
    |- include partial.liquid
    |- layout parent.liquid
        |- then include partial2.liquid

Each node will inherit scopes set by its parent nodes in this tree, lower parent will hide higher parent's variables with same name. Though render is special in this, template introduced by render can only access the variables passed to the tag explicitly. There should be no difference in this between assigned variables and variables in initial scope, no matter whether the variable is set to nil or anything else.

There maybe a bug for assigned variables here, feel free if you can reproduce.