Fix various uninitialized member pointers

Using this command:
```
find -name "thirdparty" -prune -o -name "*.h" -exec sed -i {} -e '/return /! s/\t\([A-Za-z0-9_]* \*[A-Za-z0-9_]*\)\;/\t\1 = nullptr;/g' \;
```

And then reviewing the changes manually to discard the ones that don't
seem correct/safe/good (notably changes to `core` unions).

editor/plugins/control_editor_plugin.h

@@ -186,7 +186,7 @@ public:
 class SizeFlagPresetPicker : public ControlEditorPresetPicker {
 	GDCLASS(SizeFlagPresetPicker, ControlEditorPresetPicker);
 
-	CheckBox *expand_button;
+	CheckBox *expand_button = nullptr;
 
 	bool vertical = false;