Fix exit code for pytest in case of no matched files

[tobiasdiez]

As reported in #31103 comment:44, sage -t xyz fails with exit code 5 if xyz matches no pytest tests. This is fixed.

CC: @mkoeppe @saraedum

Component: build

Author: Tobias Diez

Branch/Commit: 89b8754

Reviewer: Julian Rüth

Issue created by migration from https://trac.sagemath.org/ticket/32892

[saraedum]

Reviewer: Julian Rüth

[tobiasdiez]

comment:3

Thanks for the quick review!

[vbraun]

Changed branch from public/build/pytest-empty to 89b8754