Med: libcrmcommon: Don't tell vw_printw to print a NULL.

Doing so will cause weird behavior - it will look like it's printing,
but only newlines (and maybe spaces?). The cursor will be where you
expect, but there won't be any text after the NULL gets printed.

This happens anywhere we're using "out->begin_list(out, NULL, NULL,
NULL)" to create a new level of indentation without printing a header,
but was first discovered in printing out bundle details.