I started putting a helpful comment above the ifs as a seperator to cope with that.

public Value? Func(String arg) { 
  // Sanitize.
  if (arg.IsEmpty()) return null; 
  if (this.Bar == null) return null; 
  // Get [that] and/or do [this].
  var foo = this.baz.foo;
  ...
  return new Value();
}