tests/unittests: add empty list clist_foreach case
tests/unittests: add empty list clist_foreach case

Add clist_foreach empty list test case which should:

* not call `func`
* return `NULL`

mfrey/RIOT,mfrey/RIOT,BytesGalore/RIOT,kYc0o/RIOT,biboc/RIOT,authmillenon/RIOT,ant9000/RIOT,BytesGalore/RIOT,smlng/RIOT,x3ro/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,gebart/RIOT,rfuentess/RIOT,authmillenon/RIOT,kbumsik/RIOT,basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,mfrey/RIOT,authmillenon/RIOT,BytesGalore/RIOT,yogo1212/RIOT,gebart/RIOT,rfuentess/RIOT,cladmi/RIOT,ant9000/RIOT,toonst/RIOT,RIOT-OS/RIOT,neiljay/RIOT,neiljay/RIOT,OTAkeys/RIOT,yogo1212/RIOT,miri64/RIOT,authmillenon/RIOT,josephnoir/RIOT,lazytech-org/RIOT,cladmi/RIOT,smlng/RIOT,rfuentess/RIOT,rfuentess/RIOT,rfuentess/RIOT,jasonatran/RIOT,kbumsik/RIOT,basilfx/RIOT,kYc0o/RIOT,aeneby/RIOT,yogo1212/RIOT,biboc/RIOT,kbumsik/RIOT,biboc/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,A-Paul/RIOT,x3ro/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,mfrey/RIOT,kYc0o/RIOT,josephnoir/RIOT,neiljay/RIOT,mtausig/RIOT,authmillenon/RIOT,avmelnikoff/RIOT,biboc/RIOT,basilfx/RIOT,x3ro/RIOT,aeneby/RIOT,OlegHahm/RIOT,kbumsik/RIOT,toonst/RIOT,cladmi/RIOT,A-Paul/RIOT,OlegHahm/RIOT,mtausig/RIOT,avmelnikoff/RIOT,basilfx/RIOT,josephnoir/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,authmillenon/RIOT,toonst/RIOT,miri64/RIOT,josephnoir/RIOT,x3ro/RIOT,yogo1212/RIOT,kaspar030/RIOT,cladmi/RIOT,mtausig/RIOT,kbumsik/RIOT,A-Paul/RIOT,lazytech-org/RIOT,mtausig/RIOT,aeneby/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,josephnoir/RIOT,mtausig/RIOT,smlng/RIOT,jasonatran/RIOT,neiljay/RIOT,cladmi/RIOT,biboc/RIOT,avmelnikoff/RIOT,OlegHahm/RIOT,gebart/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,yogo1212/RIOT,avmelnikoff/RIOT,toonst/RIOT,neiljay/RIOT,aeneby/RIOT,kYc0o/RIOT,smlng/RIOT,lazytech-org/RIOT,lazytech-org/RIOT,miri64/RIOT,A-Paul/RIOT,aeneby/RIOT,basilfx/RIOT,gebart/RIOT,jasonatran/RIOT,x3ro/RIOT,mfrey/RIOT,avmelnikoff/RIOT,toonst/RIOT,smlng/RIOT,gebart/RIOT,BytesGalore/RIOT,yogo1212/RIOT,A-Paul/RIOT,jasonatran/RIOT