Fix issue with scanning subfolders if ignoreDirectories is not used

[Finholt]

I noticed that if I did not set ignoreDirectories...

[plugins.inputs]
  checkPaths = ['/']
  # ignoreDirectories = ['/admin']
  resultMode = "warn"

...then none of the subfolders were being scanned. This appears to be because directoryFilter can be used both exclusively and inclusively.

When used with ignoreDirectories, it was being interpreted as "exclude these paths" (as expected), but if it was not set, it was being interpreted as "include no paths"

[mxmason]

Hi, @Finholt! Thank you so much for writing this fix, and your patience with us in getting back to you!