Fix `modifyMtime` task when `generateSmithyBuild` task is up to date (#1471)

When `generateSmithyBuild` task is up to date, the hashes of the build
are not calculated, so `modifyMtime` fails, because it expects them to
be registered in Gradle's project properties. This can happen if you run
a command like `./gradlew codegen-server-test:test` twice consecutively.

In this case, we should skip the `modifyMtime` task altogether.
Everything is up to date, so no codegen will run, and no artifacts will
change.