Fix NullReferenceException loop in AdventurerPlateStep

[jblemee]

Problem

AdventurerPlateStep.NeedsUpdateEveryFrame() returns true so Run() is invoked every frame while the Adventurer Plate window is open. On the very first frames after the window opens (and apparently on some 7.5 sessions in general), AgentCharaCard.Instance()->Data->PortraitTexture or its D3D11Texture2D pointer can still be null, but GetCurrentCharaViewImage dereferences both unconditionally.

The result is a tight loop of System.NullReferenceException thrown from GetCurrentCharaViewImage, swallowed by Dalamud's per-event PluginErrorHandler, and printed as [ERR] once per frame in dalamud.log — the step also never reaches Completed?.Invoke() until the texture happens to be ready.

[ERR] [Auracite] Exception in event handler IFramework::Update
System.NullReferenceException: Object reference not set to an instance of an object.
   at Auracite.AdventurerPlateStep.GetCurrentCharaViewImage() in …/AdventurerPlateStep.cs:line 106
   at Auracite.AdventurerPlateStep.Run() in …/AdventurerPlateStep.cs:line 36

Change

• Walk the AgentCharaCard.Instance()->Data->PortraitTexture->D3D11Texture2D pointer chain step by step inside GetCurrentCharaViewImage, returning Image? and bailing out at the first null link.
• In Run(), also check storage for null and bail out when GetCurrentCharaViewImage returns null. The next frame retries.

When the texture is ready the existing happy-path runs unchanged and Completed fires exactly once. The error-spam in dalamud.log goes away on slow open.

Testing

Built and tested live on a 7.5 client (FR, API 15). Opening the Adventurer Plate now produces no [ERR] lines for Auracite and the step advances cleanly to the next stage. Verified the export still produces valid base-plate.png, backing.png, etc. in the resulting archive.