A kind ruling, but by stricter RAW your half-orc was dead.
- There aren't any hit points available to have 1 HP
- You were "killed outright"
From MM p. 302 on the Life Drain skill.
This reduction lasts until the target finishes a long rest. The
target dies if this effect reduces its hit point maximum to 0.
From PHB p. 41 on Relentless Endurance
When you are reduced to zero hit points but not killed outright, you
can drop to 1 hit point.
Technically, you were killed outright since the rule says "the target dies" so you were dead (not just reduced to zero hit points) as a result of the wraith's Life Drain skill. This is a specific case (target dies at 0HP and 0 max HP) rather than the general case of death saves / stabilization chances before dying.
PHB p. 197 on Dropping to 0 Hit Points
When you are reduced to 0 HP you either die outright or fall unconscious //snip// if damage reduces to 0 HP and fails to kill you you fall unconscious.
As written, the Life Drain skill killed you outright.
The other point is that if your max hit points are zero, they can't be 1 since 0 is the maximum value and 1 is greater than that.
All the above said, I like the ruling for its cinematic and heroic overtones: "he kept fighting because he didn't know he was dead" seems a good fit for a half-orc in the heat of battle. (Not to mention that scene in Fatal Attraction where Glenn Close (dead?) lunges up and out of the bathtub to attack one more time).