fix: filter out {} stages from the view pipeline (COMPASS-4977)

[lerouxb]

There are various cases where generateStage returns {} so the pipeline array has valid stages with empty objects mixed in. Solution was to just filter those out where they get made in openCreateView.

I split the code out so I can easily unit test it as openCreateView suffers from that thunk dispatch problem where it isn't easy to spy on what it passes on.

[gribnoysup]

Maybe we can change generateStage in a way that returns something that is easier to identify as an empty stage (like null or something) and then we don't need this dependency on isEmpty to do the check (lodash is used a lot across the codebase, but we are trying to use it less recently and remove it where appropriate), wdyt?

[lerouxb]

I considered that and had a look at the other places (outside of tests) that call generateStage():

• compass/packages/compass-aggregations/src/utils/stage.js

  Line 27 in fe3c919

  const s = generateStage(stage);

• compass/packages/compass-aggregations/src/modules/update-view.js

  Line 63 in fe3c919

  const viewPipeline = state.pipeline.map((p) => (p.executor || generateStage(p)));

• compass/packages/compass-aggregations/src/modules/pipeline.js

  Line 516 in fe3c919

  results.push(stage.executor || generateStage(stage));

• compass/packages/compass-aggregations/src/modules/pipeline.js

  Line 565 in fe3c919

  stage.executor = generateStage(stage);

We'd have to change those to turn null back into {} because that seems to be what they expect. So I'm not sure - it seems debatable either way.

[gribnoysup]

Makes sense, thanks for the context!