[5.x] Fix null coalescence operator evaluation

[godismyjudge95]

@marcorieser Brought up in discord that the null coalescence operator is not functioning like you'd expect.

It is expected that if the left side of the operator is not null then the right side would never be evaluated.

This is a contrived example that showcases the behavior:

{{ hello = "Hello" }}
{{ world = "World" }}
{{ hello ?? (world = "Earth") }} {{ world }}

Based on the order of operations and evaluation order, you'd expect the above to output:

Hello World

But it instead outputs:

Hello Earth

Because the right side of the operator is evaluated regardless of the left side being nullish.

This PR adds a test and fixes that behavior.