Rendering title without subtitle through title command crashes

[sciwhiz12]

Minecraft Version: 1.16.1

Forge Version: 32.0.47

Logs: Crash report (ignore the TEST2.zip pack, it's out of scope of this issue)

Steps to Reproduce:

1. /title @a clear (makes sure that title and subtitle are clear)
2. /title @a title "Hello"

Description of issue:
When running the title @a title <text> command (any player can be in place of @a, only those players will crash) to render a title on-screen without a previous call to title @a subtitle <text> to record a subtitle to be rendered alongside it, a crash occurs.

The cause is in ForgeIngameGui#renderTitle, the Forge replacement for IngameGui, which does not include null checks for this.displayedSubTitle and this.displayedTitle, which means that not having a recorded subtitle will cause a crash.
displayedTitle should not really be affected, since the title @a title <text> command sets its value and then renders the title immediately after.
IngameGUI has both of these null checks present.

Original bug report on forums

[diesieben07]

#6907