GDScript: Initialize static variables with defaults in-editor

When the script is not marked as `@tool` the static constructor is not
called and thus the variables contain `null` by default. But since some
validated operations requires a valid value, this would cause a crash.

This commit solves this by initializing the static variables with a
default value based on their types in the editor, when they are not
marked as `@tool`, so if some `@tool` script access them, they will have
a valid typed value, avoiding the crash.

modules/gdscript/gdscript.h

@@ -169,6 +169,9 @@ private:
 	GDScriptFunction *static_initializer = nullptr;
 
 	Error _static_init();
+#ifdef TOOLS_ENABLED
+	void _static_default_init(); // Initialize static variables with default values based on their types.
+#endif
 
 	int subclass_count = 0;
 	RBSet<Object *> instances;