Fix busy waiting on stdin

When running an execution, Nomad continuously reads from the stdin
reader. Because the readers we implemented (codeOceanToRawReader and
nullReader) return zero if there is no input available, this leads to
busy waiting and a high CPU load on Poseidon. By waiting indefinitely in
case of the nullReader and for at least one byte on case of the
codeOceanToRawReader before returning, we prevent this issue.

nomad/nomad.go

@@ -292,7 +292,8 @@ func (a *APIClient) LoadEnvironmentJobs() ([]*nomadApi.Job, error) {
 type nullReader struct{}
 
 func (r nullReader) Read(_ []byte) (int, error) {
-	return 0, nil
+	// An empty select blocks forever.
+	select {}
 }
 
 // ExecuteCommand executes the given command in the given allocation.