fix(AI): Add null checks for training orders and factory lookup

[seer-by-sentry]

Fixes CLIENT-44Z. The issue was that: Access violation in ThingTemplate::isEquivalentTo due to this pointer being invalid, likely from a use-after-free of a ThingTemplate object.

• Added null check for 'order->m_thing' in 'AIPlayer::startTraining' to prevent dereferencing a null pointer.
• Added null check for 'thing' parameter in 'AIPlayer::findFactory' to prevent dereferencing a null pointer.
• Ensures robustness when AI attempts to train or find factories for invalid (null) unit templates.

This fix was generated by Seer in Sentry, triggered automatically. 👁️ Run ID: 12233138

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