Precompiled libraries: only select a folder if it contains files

[facchinm]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

If a folder meant to contain precompiled libraries is empty (or only contains other folders) it can be safely skipped

• What is the current behavior?

The folder is not skipped, so libraries with src/$architecture/$fp_specs will try to use src/$architecture/ as precompiled base folder, even if it doesn't contain any library

• What is the new behavior?

The folder is skipped and the library can be compiled from sources

• Does this PR introduce a breaking change, and is
  titled accordingly?

• Other information:

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[silvanocerza]

Python tests are failing:

 poetry run flake8
./test/test_compile.py:244:1: E302 expected 2 blank lines, found 1
task: Failed to run task "python:check": exit status 1

You should run task python:format locally to fix the issue, to be sure it's fixed then run task python:check.

[silvanocerza]

@facchinm nevermind, I and @cmaglie pushed the fixes. :)

[silvanocerza]

We pushed some fixes to the failing steps of test_compile.py::test_compile_without_precompiled_libraries, now there are other steps that are failing that we are investigating.