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/messages.ts

@@ -1770,7 +1770,7 @@ export const generateWAMessageFromContent = (
 		const innerContent = innerMessage[key as Exclude<keyof proto.IMessage, 'conversation'>]
 		const contextInfo: proto.IContextInfo =
 			(innerContent && typeof innerContent === 'object' && 'contextInfo' in innerContent && (innerContent as Record<string, unknown>).contextInfo as proto.IContextInfo) || {}
-		contextInfo.participant = jidNormalizedUser(participant ?? '')
+		contextInfo.participant = jidNormalizedUser(participant!)
 		contextInfo.stanzaId = quoted.key.id
 		contextInfo.quotedMessage = quotedMsg