minor: null safety checks added (typically only really relevant in case
of misuse of ManagedProcess API intended Lifecycle - as e.g done in the
ManagedProcessTest - waitFor...() should never cause NPE - even if not
start()'d.

Signed-off-by: Michael Vorburger <mike@vorburger.ch>
TODO reminder to later look into closing process STD* channels

Signed-off-by: Michael Vorburger <mike@vorburger.ch>