Make unit_ishostile always return a non-nil value

Have been messing with this function in the dfhack repl and when it returns nil it's hard to know what that means unless you have also already read the code. So this handles a couple spots in `unit_ishostile` that previously just returned false-ey `nil`s to return `false`

plugins/ruby/unit.rb

@@ -134,7 +134,7 @@ module DFHack
         # does not include ghosts / wildlife
         def unit_ishostile(u)
             # return true if u.flags3.ghostly and not u.flags1.inactive
-            return unless unit_category(u) == :Others
+            return false unless unit_category(u) == :Others
 
             case unit_other_category(u)
             when :Berserk, :Undead, :Hostile, :Invader, :Underworld
@@ -152,6 +152,8 @@ module DFHack
                     case unit_checkdiplomacy_hf_ent(histfig, group)
                     when 4, 5
                         true
+                    else
+                        false
                     end
 
                 elsif diplo = u.civ_tg.unknown1b.diplomacy.binsearch(df.ui.group_id, :group_id)