use_crate fails when no features specified

[JosiahParry]

          This change broke the function. Looks like we might be missing a test for this. I installed the new version 

rextendr::use_crate("lazy-static")
Running cargo add lazy-static '--features ' '--optional false'
Error:
! ! System command 'cargo' failed

This is because "--features " is included in the vector that is passed to processx. --features

_Originally posted by @JosiahParry in https://github.com/extendr/rextendr/pull/361#discussion_r1764103981_

[CGMossa]

Fixes by #388 (thanks @kbvernon for both the PR, and for notifying me that this issue can now be closed)