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

[aignas]

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.