pre-compile is failing for binary targets if there is no main specified

With #2768 optional, it is possible to have zero `.py` files or the `main` is not used or specify `main_module`.

Then I get errors:
```
providers = py_executable_impl(
File "/home/ignas-anikevicius/.cache/bazel/_bazel_ignas-anikevicius/e1764a0ec8835154284d951e9bb11b85/external/rules_python+/python/private/py_executable.bzl", line 256, column 35, in py_executable_impl
return py_executable_base_impl(
File "/home/ignas-anikevicius/.cache/bazel/_bazel_ignas-anikevicius/e1764a0ec8835154284d951e9bb11b85/external/rules_python+/python/private/py_executable.bzl", line 1049, column 50, in py_executable_base_impl
main_py = precompile_result.py_to_pyc_map[main_py]
Error: key None not found in dictionary
```

It seems that we may have forgotten to handle the `main_module` in precompilation.

Comienza en python/private/py_executable.bzl alrededor de py_executable_impl y py_executable_base_impl, especialmente en la búsqueda de precompilación de la línea 1049. Reproduce un destino binario sin archivos .py y sin especificar main ni main_module; después, verifica que la precompilación gestione el punto de entrada seleccionado sin buscar None en py_to_pyc_map.

Escrito por el modelo de indexación a partir del texto del issue.