v5: Don't kill the chainloader when Unity log callbacks are stripped (#1392)

UnityLogSource's static constructor falls back to
Application.RegisterLogCallback when Application.logMessageReceived is
absent. Unity's managed code stripper can remove both, in which case
GetMethod returns null and the unguarded Invoke throws a
NullReferenceException out of the static constructor.

That exception propagates through Chainloader.Initialize, so the
chainloader never starts and no plugins load. The failure is close to
undiagnosable: LogOutput.log ends at "Preloader finished" with no error,
and the exception only appears on stdout.

Unity log forwarding is optional, so warn and continue instead.

Backport of the same fix proposed for master in #1389.

BepInEx/Logging/UnityLogSource.cs

@@ -59,6 +59,20 @@ namespace BepInEx.Logging
 			else
 			{
 				MethodInfo registerLogCallback = typeof(Application).GetMethod("RegisterLogCallback", BindingFlags.Public | BindingFlags.Static);
+
+				// Unity's managed code stripper can remove both Application.logMessageReceived and the
+				// legacy Application.RegisterLogCallback. Invoking a null MethodInfo here throws out of
+				// this static constructor, which propagates through Chainloader.Initialize and silently
+				// prevents the chainloader (and therefore every plugin) from starting. Unity log
+				// forwarding is optional, so degrade instead of taking the whole loader down.
+				if (registerLogCallback == null)
+				{
+					Logger.LogWarning("Unity log forwarding is unavailable: neither Application.logMessageReceived " +
+					                  "nor Application.RegisterLogCallback exist in this build (they were most likely " +
+					                  "stripped). Unity log messages will not appear in the BepInEx log.");
+					return;
+				}
+
 				registerLogCallback.Invoke(null, new object[] { callback });
 				//UnsubscribeAction = () => registerLogCallback.Invoke(null, new object[] { null });
 			}