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/WAM/encode.ts

@@ -78,7 +78,11 @@ function encodeEvents(binaryInfo: BinaryInfo) {
 			}
 
 			const fieldFlag = extended ? FLAG_EVENT : FLAG_FIELD | FLAG_EXTENDED
-			binaryInfo.buffer.push(serializeData(id!, value, fieldFlag))
+			if (id == null) {
+			continue
+		}
+
+		binaryInfo.buffer.push(serializeData(id, value, fieldFlag))
 		}
 	}
 }