Mono/C#: Don't try to load project assembly in project manager

This was causing an error message when launching the manager,
because there is no project assembly to load.

(cherry picked from commit d7e21ecd63a984054c6bd330ee8ff6de5e53de7d)

modules/mono/mono_gd/gd_mono.cpp

@@ -427,6 +427,9 @@ void GDMono::initialize_load_assemblies() {
 	CRASH_COND_MSG(!tool_assemblies_loaded, "Mono: Failed to load '" TOOLS_ASM_NAME "' assemblies.");
 #endif
 
+	if (Main::is_project_manager())
+		return;
+
 	// Load the project's main assembly. This doesn't necessarily need to succeed.
 	// The game may not be using .NET at all, or if the project does use .NET and
 	// we're running in the editor, it may just happen to be it wasn't built yet.