ChallengeLoadScreen: Add safety checks for NULL generalOpponent

[seer-by-sentry]

Fixes CLIENT-HG. The issue was that: Null GeneralPersona pointer dereferenced during AudioEventRTS construction, causing fatal read access violation (0x64).

• Added null checks for generalOpponent in ChallengeLoadScreen::activatePieces, ChallengeLoadScreen::activatePiecesMinSpec, and ChallengeLoadScreen::setupLoadScreen to prevent potential crashes when the opponent general data is missing.
• Added a check in ChallengeLoadScreen::setupLoadScreen to log an error if the GeneralPersona for the opponent cannot be found, and return early to prevent further operations with a null pointer.
• Added a null check before playing the opponent's taunt sound to avoid crashes if generalOpponent is null.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 2014261

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