fix(types): remove non-null assertions in remaining 12 files

Files fixed:
- messages-media.ts: stream/buffer guards, file path validation
- decode-wa-message.ts: message field guards, optional chaining
- version-cache.ts: narrowing after null checks
- jid-utils.ts: split result guards, user fallbacks
- communities.ts: attrs fallbacks with ?? operator
- sticker-pack.ts: response guards
- business.ts: attrs guards
- socket.ts: onClose guard, pairingCode guard, creds.me guard
- event-buffer.ts: null guards
- signal.ts: null guards
- encode.ts (WAM): id null check with continue
- upstream history.ts: conversations/pushnames null guards

All 26 files across the project are now corrected.
Total: ~248 non-null assertions replaced with proper null guards.

https://claude.ai/code/session_01E2cfX1N3sJgCJBTvzGazSG

src/Utils/sticker-pack.ts

@@ -575,7 +575,7 @@ export const prepareStickerPackMessage = async (
 			duplicateCount++
 
 			// Merge emojis (deduplicate)
-			const mergedEmojis = Array.from(new Set([...existingMetadata.emojis!, ...emojis]))
+			const mergedEmojis = Array.from(new Set([...(existingMetadata.emojis ?? []), ...emojis]))
 			existingMetadata.emojis = mergedEmojis
 
 			// Merge accessibility labels (concatenate with separator if both exist)