VisualScript call nodes should have a default arg of zero by default

**Godot version:**
3.1.1

**Issue description:**
When adding a method node the default args are often not 0 and arguments are hidden, which isn't very intuitive. All arguments should be visible by default to show beginners everything, as they could get confused because an argument is missing (Seen in Godot Discord).

**Steps to reproduce:**
Add a method call with more than one argument to a VisualScript.