When a property getter returns an empty/missing strv-array, in multi-line
mode we should not print any lines.

To get that right, we must mark the cell as STRV type, even if there is no value
provided.

Previously, with text_out_flags having NM_META_ACCESSOR_GET_OUT_FLAGS_STRV
and value being NULL, we would not set
  cell->text_format = PRINT_DATA_CELL_FORMAT_TYPE_STRV;
and thus, later on the value would be treated as a missing (plain)
string.