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.
The property getter for certain properties tries to return
a strv array.
In this case, the result should be identical, whether an
empty strv array or NULL is returned.
Let _ip_config_get_routes() return %NULL if there are no routes.
This should have no practical difference, but it actually exposes
a bug in "cli/common/utils.c", which was previously hidden by
not commonly returning %NULL. This bug will be fixed in the
next commit.
