Avoid duplicate blank lines after multiline sections

[ada-powerful]

Summary

• avoid adding an extra top-level separator when the previous rendered line is already blank
• add a regression for a multiline string followed by the next top-level section
• update the selector-filtering expected output to keep one section separator

Closes #527.

Validation

• conda env create -p ./.conda-test -f environment.yaml
• pytest tests/parser/test_recipe_reader.py tests/parser/test_recipe_variant.py -q
• pytest tests/parser -q
• git diff --check

[conda-bot]

We require contributors to sign our Contributor License Agreement and we don't have one on file for @ada-powerful.

In order for us to review and merge your code, please e-sign the Contributor License Agreement PDF. We then need to manually verify your signature, merge the PR (conda/infrastructure#1331), and ping the bot to refresh the PR.

[schuylermartin45]

This has all the tell-tale signs of an LLM bot, not to mention the near-identical PR #529 filed a few hours later, I'm closing this.

I guess this is what I get for using good-first-issue these days...