fix: revert remaining ?? '' patterns across Utils, WABinary, Signal, and WAUSync files

Completes the comprehensive revert of defensive null coalescing patterns
introduced in PRs 124-129. These patterns (e.g. `value ?? ''`) silently
produced empty strings instead of failing fast, causing subtle bugs like
malformed JIDs and broken Signal session lookups.

Files: libsignal.ts, messages-recv.ts, chat-utils.ts, generics.ts,
history.ts, messages-media.ts, messages.ts, process-message.ts,
validate-connection.ts, jid-utils.ts, UsyncBotProfileProtocol.ts

https://claude.ai/code/session_01XaA7GwNaB6azTHFYQ8WEpB

src/Utils/history.ts

@@ -294,7 +294,7 @@ export const processHistoryMessage = (item: proto.IHistorySync, logger?: ILogger
 		case proto.HistorySync.HistorySyncType.FULL:
 		case proto.HistorySync.HistorySyncType.ON_DEMAND:
 			for (const chat of (item.conversations ?? []) as Chat[]) {
-				const chatId = chat.id ?? ''
+				const chatId = chat.id!
 
 				// Source 2: Extract LID-PN mapping from conversation object
 				// This handles cases where the mapping isn't in phoneNumberToLidMappings
@@ -371,7 +371,7 @@ export const processHistoryMessage = (item: proto.IHistorySync, logger?: ILogger
 			break
 		case proto.HistorySync.HistorySyncType.PUSH_NAME:
 			for (const c of (item.pushnames ?? [])) {
-				contacts.push({ id: c.id ?? '', notify: c.pushname ?? '' })
+				contacts.push({ id: c.id!, notify: c.pushname! })
 			}
 
 			break