"无人委托诉讼的" can be translated as "where there is no one who has entrusted [someone] with litigation" or more naturally in legal contexts as "where no one has authorized legal representation (for the case)".
A more concise and contextually appropriate translation, depending on the specific legal document or discussion, might be:
"Where there is no authorized litigation representative"
or, if emphasizing the lack of any party initiating the suit:
"Where no party has initiated litigation (through an authorized representative)"
For a more literal and general translation:
"When there is no one who has commissioned the lawsuit"
However, in legal English, the focus is often on the authorization or representation aspect rather than the simple act of commissioning, so the first or second suggestions may be more fitting in many cases.