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/validate-connection.ts

@@ -237,7 +237,7 @@ export const configureSuccessfulPairing = (
 				content: [
 					{
 						tag: 'device-identity',
-						attrs: { 'key-index': String(deviceIdentity.keyIndex ?? '') },
+						attrs: { 'key-index': deviceIdentity.keyIndex!.toString() },
 						content: accountEnc
 					}
 				]