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.

Start in python/private/py_executable.bzl around py_executable_impl and py_executable_base_impl, especially the precompilation lookup at line 1049. Reproduce a binary target with no .py files, no main, or main_module specified, then verify that precompilation handles the selected entry point without looking up None in py_to_pyc_map.

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