Guard against nil account in createOrUpdateAccountWithAuthToken callback

[jkmassel]

Follows the same pattern as #25964.

Summary

• AccountService.createOrUpdateAccountWithAuthToken:success:failure: declares its success block parameter WPAccount * _Nonnull, but the implementation resolves the account via existingObjectWithID:error:, which can return nil.
• Swift imports the callback as non-optional WPAccount, so the single caller (WordPressComSyncService) cannot guard it.

Root Cause

AccountService.m passes the result of existingObjectWithID:error: straight into the _Nonnull success block. If the object can't be re-resolved on the main context (a Core Data race), account is nil. WordPressComSyncService.syncOrAssociateBlogs(account:) then reads the Swift-native isDefaultWordPressComAccount on it — risking a trap or wrong-account behavior in the login/sync path.

Fix

• Mark the success block parameter WPAccount * _Nullable in AccountService.h/.m so the annotation is honest. The implementation body already passes nil legally.
• Guard the account in WordPressComSyncService.syncWPCom(...); on nil, fail through the existing onFailure path.

Test plan

• WordPress app builds (generic iOS Simulator).
• SwiftLint clean.
• Signing in to a WordPress.com account still syncs the account and its blogs.

Part of an audit of Objective-C non-null declarations whose implementations can return nil; see #25964 for the first fix in the series.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	34120
	Version	PR #25967
	Bundle ID	org.wordpress.alpha
	Commit	514a904
	Installation URL	3iges7j0uls9g

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	34120
	Version	PR #25967
	Bundle ID	com.jetpack.alpha
	Commit	514a904
	Installation URL	515ada6pm29v0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.