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/editor_properties_array_dict.h

@@ -151,7 +151,7 @@ class EditorPropertyDictionary : public EditorProperty {
 	Button *edit = nullptr;
 	MarginContainer *container = nullptr;
 	VBoxContainer *property_vbox = nullptr;
-	EditorSpinSlider *size_sliderv;
+	EditorSpinSlider *size_sliderv = nullptr;
 	Button *button_add_item = nullptr;
 	EditorPaginator *paginator = nullptr;