Check for missing or empty filetype in syntax files

To avoid surprises like with jsonnet.

pkg/highlight/parser.go

@@ -209,11 +209,19 @@ func ParseFile(input []byte) (f *File, err error) {
 		if k == "filetype" {
 			filetype := v.(string)
 
+			if filetype == "" {
+				return nil, errors.New("empty filetype")
+			}
+
 			f.FileType = filetype
 			break
 		}
 	}
 
+	if f.FileType == "" {
+		return nil, errors.New("missing filetype")
+	}
+
 	return f, err
 }