fix: empty override tables no longer wipe base DB spawns; real-time pin refresh on NPC learn

Two fixes for QuestieLearner real-time pin rendering:

1. _MergeOverride now skips empty tables (IsEmptyTable guard).
   QuestieLearner stores spawns={} before any coords are captured.
   Previously _MergeOverride treated {} as valid data and replaced the
   base DB's real spawn coordinates with an empty table, breaking pins
   for all NPCs that had been seen but not killed (mouseovers, etc.).

2. _InvalidateSpawnListsForNPC triggers after LearnNPC adds new spawn data.
   The quest objective system caches spawnList once per objective.
   When a kill adds new coordinates, the cached list is stale.
   The new helper:
   - Scans all active quest objectives for references to the NPC
   - Clears objective.spawnList and objective.AlreadySpawned
   - Calls QuestieQuest:UpdateQuest to rebuild pins with fresh data
   This makes learned pins appear in real-time without /reload.

Database/QuestieDB.lua

@@ -15,12 +15,19 @@ local _QuestieDB = QuestieDB.private
 --   1. override[stringKey]  (string-keyed, e.g. from wotlkNPCFixes)
 --   2. override[intKey]     (numeric-keyed, e.g. from QuestieLearner / AscensionDB)
 --   3. rawdata[intKey]      (fallback to compiled DB)
+--
+-- Empty tables from QuestieLearner (e.g. spawns={}) are treated as "not present"
+-- so the base DB's real spawn data is preserved until actual coordinates are learned.
 ------------------------------------------------------------------------
+local function IsEmptyTable(val)
+    return type(val) == "table" and next(val) == nil
+end
+
 local function _MergeOverride(result, override, rawdata, keyMap)
     for stringKey, intKey in pairs(keyMap) do
         if override[stringKey] ~= nil then
             result[stringKey] = override[stringKey]
-        elseif override[intKey] ~= nil then
+        elseif override[intKey] ~= nil and not IsEmptyTable(override[intKey]) then
             result[stringKey] = override[intKey]
         elseif rawdata then
             result[stringKey] = rawdata[intKey]