Exit code is non-zero when no namespaces are linted and caching is enabled

[kjothen]

Linting a large monorepo is time-consuming in CI builds. This can be mitigated by caching the .eastwood file between runs on the same branch. However, eastwood returns a non-zero exit code when no namespaces were linted, which is problematic. Rather than ignore the error code and continue the build, it would be better to return a zero error code in this circumstance.