Fix: Handle empty py_library targets

Fixes an edge-case like the following, which in 0.0.9 failed when the Aspect was run on it:

py_library(
    name = "foo",
    srcs = [],
    deps = [":bar"],
)

The error seen was:

The following files have no generating action:
test/empty_srcs_lib_dummy_out
test/empty_srcs_lib_mypy_exe