cmd_is_installed fails if no utils defined

[snystrom]

Because cmd_is_installed calls util = TRUE, if no utils, get error. This really highlights how this is a bad design choice to begin with #22.

my_fun <- cmd_search_path("~/my/path")
is_installed <- cmd_is_installed(my_fun)
is_installed()

Error in util == TRUE : 
  comparison (1) is possible only for atomic and list types

Tests to add

• [ ] path search w/ no utils -> is_installed()

[snystrom]

Hotfix could be if util == TRUE -> return everything even if no utils?