Make sure 'simpleclip-custom-content-provider is non-nil before using it.

[djl]

Fixes an issue introduced in #15.

Before this fix 'simpleclip-custom-content-provider was always used because it only checked that the variable was bound instead of checking for a non-nil value.

(boundp 'simpleclip-custom-content-provider)
;; t
simpleclip-custom-content-provider
;; nil

This resulted in always running an empty shell command.

A cleaner way might to be to only check for a non-nil value since if it's non-nil, 'bound-p will return true.

[rolandwalker]

Hi!

Makes sense.