check for nil testsuite when parsing test results

[tylersouthwick]

If a project has no tests and neotrest tries to run tests, no tests are found.
If a project has tests, and the tests are removed, and then neotest tries to run tests, it gets the following error:

neotest-rust: ~/.local/share/nvim/lazy/neotest/lua/nio/init.lua:105: The coroutine failed with this message:                                                                 
...l/share/nvim/lazy/neotest-rust/lua/neotest-rust/init.lua:362: attempt to get length of field 'testsuite' (a nil value)                                                              
stack traceback:                                                                                                                                                                       
        ...l/share/nvim/lazy/neotest-rust/lua/neotest-rust/init.lua: in function 'results'                                                                                             
        ...al/share/nvim/lazy/neotest/lua/neotest/client/runner.lua:131: in function '_run_spec'                                                                                       
        ...al/share/nvim/lazy/neotest/lua/neotest/client/runner.lua:89: in function <...al/share/nvim/lazy/neotest/lua/neotest/client/runner.lua:88>  

this fixes #32 and adds unit test to the change in #33

[rouge8]

This seems reasonable. Please run pre-commit to format the files and I’ll approve it!

[tylersouthwick]

This seems reasonable. Please run pre-commit to format the files and I’ll approve it!

ran pre-commit and rebased

[rouge8]

Thanks!