Fix trailing undefined for optional parameters not omitted with @send

[cknitt]

Fixes #6715

[zth]

Nice find! I wonder if there are more code paths where it should be applied...

[cknitt]

@zth I now moved the handling directly to assemble_args_no_splice so it applies to more cases, e.g., @new. @get/@set/@get_index/@set_index also use this function, but I think it doesn't really matter there.

Not sure what the cases Js_module_as_fn and Js_module_as_class are.

[cknitt]

Also, I was wondering: External function calls with trailing undefineds will probably be the exception rather than the rule, but we are now doing additional list allocations whenever we emit code for any external function call - should we maybe first check if there are any trailing undefineds before doing these list computations?

[zth]

I'll let @cristianoc have a final look at this too, looks good from my side.

[cknitt]

Added check has_undefined_trailing_args before during the List.rev stuff.