NR S12: hide empty sides

There has been a report that some scenario would be nearly unbeatable if they
are empty in the first place, so this has the side effect of not showing the
player that he misses something. (Nevertheless, they should be hidden)

[ci skip]

(cherry-picked from commit 47c3947d0aa651fd32545d6292d5cfed9fd02864)

data/campaigns/Northern_Rebirth/scenarios/12a_Get_the_Gold.cfg

@@ -293,6 +293,11 @@
                     y=3,4,5,3,4,5,3,4,5
                     terrain=Gs^Fp
                 [/terrain]
+
+                [modify_side]
+                    sie=4
+                    hidden=yes
+                [/modify_side]
             [/then]
         [/if]
 
@@ -354,6 +359,12 @@
                 {CLEAR_VARIABLE place_eryssa_at}
                 {CLEAR_VARIABLE eryssa_starts_at}
             [/then]
+            [else]
+                [modify_side]
+                    sie=5
+                    hidden=yes
+                [/modify_side]
+            [/else]
         [/if]
 
         # Recall heroes