services: zathura: Add serialize-hex-color to avoid warnings

modules/radix/home/services/zathura.scm

@@ -110,6 +110,9 @@
     (null? (filter (negate hex-digit?) rest)))
    (else #f)))
 
+(define serialize-hex-color              ; Just to avoid warnings
+  (@@ (gnu services configuration) empty-serializer))
+
 (define-maybe hex-color)
 
 (define-configuration/no-serialization face