Creatures immune to the conditions subtype do not receive the condition at all.. The Emerald Tegu rolls his effect die, but it his Rot token cannot be placed on the Necromancer.
Immunity prevents them from being a legal recipient.
Immunity, codes pg 41: This object is immune to all attacks, damages, CONDITIONS and EFFECTS of the specifed damage type.
Burns are a Flame effect and Flame condition (as defined in their codex entry), a creature who is Flame Immune, or Burnproof cannot actually recieve the Burn Marker.
A Marker is never put on a creature then sit idle doing nothing.. it simply doesnt get placed. Much like if that same Emerald Tegu was attacking a poison immune Mana Crystal..it wouldnt stack up Rots on said Mana Crystal.