Bug: null-coalescing operator always returns true

The following code:
```csharp
static bool Test(Func<bool> condition) {
  return condition?.Invoke() ?? true;
}
```
always returns true, because it compiles to:
```lua
Test = function (condition)
  local default = condition
  if default ~= nil then
    default = default()
  end
  return default or true
end
```