meson: avoid passing the --features flag to wrapper when empty

Currently if you try to build only a select few plugins with Meson by using -Dauto_features=disabled and then -D[plugin]=enabled, you get an error like this if the plugins require no cargo features (because the wrapper gets passed --features despite there being none):

cargo_wrapper.py: error: argument --features: expected at least one argument

This MR fixes that by preventing Meson from passing the --features flag if there are none

Alternatively the cargo_wrapper.py script could be adjusted so that the --features flag is nargs='*' instead of nargs='+', and then passing no features after the flag should be fine

I've no preference on either solution, so does one of those solutions sound acceptable? This MR right now implements the first

Thank you!