Add FlickeringText GetReportString null handler

[Fayti1703]

This fixes a base-game issue where exceptions thrown in DisplayModule.Draw could crash the game to desktop, if the Main Menu was bypassed (e.g. by launching an exception with -extstart).

To reproduce the original issue:

1. Create an Extension with the example broken node
   1. This Extension must have an empty Missions/EmptyDirectory directory.
2. Connect to the node (connect err::mls_nosvc)
3. Wait 3 frames
4. Observe the game hard-crash

This crashes because DisplayModule.Draw calls Utils.SendThreadedErrorReport after 3 draw errors, launches a new thread to run Utils.SendErrorEmail.
While building up the email body, that method calls FlickeringTextEffect.GetReportString, which accesses FlickeringTextEffect.LinedItemTarget.IsDisposed.
If the Main Menu has not been rendered in the current game process execution, FlickeringTextEffect.LinedItemTarget is null, causing a crash.

Example Broken Node:

<Computer id="err::mls_nosvc" name="Mission Listing Server without a serviceName" type="4" ip="err::mls_nosvc">
	<LogoDaemon Name="Daemon To Prevent Crash On initDaemons()" />
	<!-- no 'name' attribute on this element -->
	<variableMissionListingServer articleFolderPath="Missions/EmptyDirectory"  />
</Computer>