Special handling for undef in _check_readonly()

[pjcj]

Sorry I don't have time for a proper bug report or PR right now, but I think we need a line like return '' unless defined $_[1]; on line 795 and a half otherwise we get the error message Usage: Internals::SvREADONLY(SCALAR[, ON]).

If the context or anything else isn't clear let me know and I'll do a proper report, but I hit this when calling $ddp->parse($v) when $v is undef in a _data_printer sub.

And thanks for DDP!