Fix pyperformance venv (w/o any subcommand) exception

[maurycy]

2026-01-26T19:29:17.403748000+0100 maurycy@gimel /Users/maurycy/work/pyperformance (fix-428-venv) % uv run --python 3.14 --with . python -m pyperformance venv
usage: pyperformance [-h] [-V] {run,show,compare,list,list_groups,compile,compile_all,upload,venv} ...

Compares the performance of baseline_python with changed_python and prints a report.

positional arguments:
  {run,show,compare,list,list_groups,compile,compile_all,upload,venv}
    run                 Run benchmarks on the running python
    show                Display a benchmark file
    compare             Compare two benchmark files
    list                List benchmarks of the running Python
    list_groups         List benchmark groups of the running Python
    compile             Compile and install CPython and run benchmarks on installed Python
    compile_all         Compile and install CPython and run benchmarks on installed Python on all branches and revisions of CONFIG_FILE
    upload              Upload JSON results to a Codespeed website
    venv                Actions on the virtual environment

options:
  -h, --help            show this help message and exit
  -V, --version         show program's version number and exit

Fixes #428