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/WABinary/jid-utils.ts

@@ -63,7 +63,7 @@ export const jidDecode = (jid: string | undefined): FullJid | undefined => {
 	const userCombined = jid.slice(0, sepIdx)
 
 	const [userAgent, device] = userCombined.split(':')
-	const [user, agent] = (userAgent ?? '').split('_')
+	const [user, agent] = userAgent!.split('_')
 
 	let domainType = WAJIDDomains.WHATSAPP
 	if (server === 'lid') {
@@ -78,7 +78,7 @@ export const jidDecode = (jid: string | undefined): FullJid | undefined => {
 
 	return {
 		server: server as JidServer,
-		user: user ?? '',
+		user: user!,
 		domainType,
 		device: device ? +device : undefined
 	}
@@ -114,7 +114,7 @@ export const isAnyPnUser = (jid: string | undefined) =>
 
 const botRegexp = /^1313555\d{4}$|^131655500\d{2}$/
 
-export const isJidBot = (jid: string | undefined) => jid && botRegexp.test(jid.split('@')[0] ?? '') && jid.endsWith('@c.us')
+export const isJidBot = (jid: string | undefined) => jid && botRegexp.test(jid.split('@')[0]!) && jid.endsWith('@c.us')
 
 export const jidNormalizedUser = (jid: string | undefined) => {
 	const result = jidDecode(jid)