fix: skip unregistered escrow UIDs when setting weights

[dylanuys]

Summary

• Unregistered image/video/audio escrow hotkeys resolve to None. Indexing the weight vector with None is a numpy broadcast that zeros every UID, so process_weights_for_netuid falls back to all-ones.
• Skip those None UIDs when building special_uids and when assigning escrow weights. Burn is unchanged and still assigned directly.
• This restores the intended 84% burn / 16% generator split instead of uniform weights.

Test plan

• Confirmed on SN34 that escrow lookups return None and burn still resolves to UID 135
• After deploy, block 9001800 and 9002160 logged Total weight sum: 1.0000, Actual burn rate: 0.8400 and set_weights on chain successfully!
• Revealed on-chain weights for UID 135: 140 non-zero dests, 84% on burn
• Confirm next tempo still reveals 84/16 (not all-ones)

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