counter: fix NPE on nullable element to_s
counter: fix NPE on nullable element to_s

Signed-off-by: Alexandre Terrasa <b30f4169e054ee7b8c243480bcfcef62416f3b08@moz-code.org>

jcbrinfo/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,privat/nit,privat/nit,privat/nit,jcbrinfo/nit,privat/nit,jcbrinfo/nit,jcbrinfo/nit,privat/nit,privat/nit,privat/nit,jcbrinfo/nit,jcbrinfo/nit