Fixing error when makeCallGraph with package with no closure functions

[jcrodriguez1989]

If makeCallGraph is called with a package name, e.g., package:magrittr it fails as this package contains some functions that are not closures:

library("CodeDepends")
library("magrittr")
makeCallGraph("package:magrittr")

# Error in makeUsageCollector(fun, ...) : only works for closures 

this happens because we are calling findGlobals with a non-closure. I have fixed this, by assigning that a non-closure function calls character(0) functions.