Core/Combat: Threat bar not displayed if the player engage after phase shift

[Killyana]

Description: If you start a combat via phase shift (.mode phase) the threat bar will not be displayed by the npc, even addons like omen cannot display the threat, other data looks good:

Debug threat:
Scarlet Preacher (GUID 85) does not threaten any units.
Threat list of Scarlet Preacher (GUID 85, SpawnID 129673):

1. Wartest (GUID 2) - threat 0.000000
   End of threat list.

Debug combat:
Combat refs: (Combat state: 1 | Manager state: 1)
[PvE] Wartest (SpawnID 0)

Steps to reproduce the problem:

1. .go c 129673
2. .gm off (you will be shift to phase 1)
3. .mod pha 4 (the npc will aggro but no threat displayed)

Branch(es): 3.3.5

TC rev. hash/commit: rev. a5e73e4