The cat has dental problems.
Cats chewing without food is likely due to foreign objects in their mouths or oral diseases such as angular cheilitis, gingivitis, tooth inflammation, swollen and painful gums. It is recommended that owners open their cats' mouths to check for foreign objects or redness and inflammation, and remove the foreign objects in time. If any abnormalities are found, the cat should be taken to the vet for treatment immediately. 
If your cat is experiencing nausea and vomiting due to gastrointestinal upset, causing it to chew on empty food, it's recommended that you give it an appropriate amount of probiotics to regulate its digestive system. Furthermore, cats may chew on empty food when they feel like vomiting. If you're unsure what's going on, take your cat to the vet for a checkup and appropriate treatment.
When raising a cat, it's essential to ensure they have a healthy diet. If possible, you can also clean their mouths regularly.
The cat might want to vomit.Common causes of vomiting in cats include: foreign bodies in the gastrointestinal tract, which can cause obstruction and vomiting if not removed promptly; spontaneous cystitis in cats, which can prevent urination and cause severe azotemia, leading to vomiting; and mediastinal tumors, which can compress the esophagus and cause vomiting.
The cat may have some kind of neurological problem.Neurological problems such as epilepsy can also cause cats to chew even when there's nothing in their mouths. Take them to a veterinary hospital immediately for treatment.
