Prevent execution of gtdbtk/summary when no bins pass QC

[dialvarezs]

This removes the .ifEmpty() of GTDBTK_CLASSIFYWF.out.summary to prevent running GTDBTK_SUMMARY when no summaries are available.
It also adds a warning when no bins passed QC thresholds.

Closes #981.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• If necessary, also make a PR on the nf-core/mag branch on the nf-core/test-datasets repository.
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).

[jfy133]

What does the subscribe do here 🤔 I never understood that operator...

[dialvarezs]

subscribe() executes a function on each output of a channel, without generating an ouput (contrary to map() that returns something)