can use string literals here now

Change-Id: Ieeead634ac23003924d74140a5b6aba2611db8d7
make FilePicker::createWithMode not crash

i.e. calc->data->XML Source->"browse to set source" calls
com_sun_star_comp_svt_FilePicker_get_implementation with its
single argument in arguments as opposed to the empty argument
expected there.

So allow the single-argument case, and pass the argument through and apply it
automatically via XInitialization::initialize in Application::createFilePicker.

I'm far from sure that this is the right solution, but it has the advantage of
working vs crashing.

Change-Id: I07c1baae7f47781920eac56763e8fd003a7b99e1