Allow for `[[startProcess]]` blocks without `modifiers` array

[avahe-kellenberger]

In some cases such as XF86MonBrightnessUp, the user does not want to press super, alt, ctrl etc.

The following block should work with modifiers commented out:

[[startProcess]]
command = "notify-send 'brightness up'"
keys = [ "XF86MonBrightnessUp" ]
# modifiers = [ "super" ]

193

[avahe-kellenberger]

Found out via testing this already works.