Properly mark "head" parameter of ActionSupport.stash_if_needed() as nullable

Fixes gitg-FATAL-CRITICAL:
    gitg_action_support_stash_if_needed: assertion 'head != NULL' failed
1 job for ricotz/vala in 8 minutes and 58 seconds
Status Name Job ID Coverage
  Test
passed flatpak #1170667
flatpak

00:08:58