[3.x] Add a failing test for rendering a partial inside a collection rendering

[jcoyne]

What are you trying to accomplish?

This is a bug report, of a regression in 3.25 (from 3.24).

We get this error when rendering a partial from within a rendered component:

NoMethodError: undefined method 'render' for nil
    lib/view_component/base.rb:228:in 'ViewComponent::Base#render'
    test/sandbox/test/collection_test.rb:35:in '...PartialRenderingComponent#call'
    lib/view_component/collection.rb:22:in 'block in ViewComponent::Collection#render_in'
    lib/view_component/test_helpers.rb:54:in 'ViewComponent::TestHelpers#render_inline'

I believe this was introduced by ce1c920

on this line here: ce1c920#diff-b69c75ffd85596e0b5fca1327059ecf3b2930f3f77004c78a268fa85e8ec8d9eL83

The old ||= silently masked a nil. The new code keys off instance_variable_defined?, so an explicitly-stashed nil is now honored.

I think we can fix this by adding:

(remove_instance_variable(...) || view_context

at base.rb:91.