switch_source needs the case to be present, even if its *_null

We were using f_null when the type is ter_str_id (should be t_*)

fixes #83613

Author: CoreyHendrey

data/json/mapgen_palettes/apartment_complex.json

@@ -88,26 +88,24 @@
     },
     "furniture": {
       "Ŧ": {
-        "switch": { "param": "fencing_type", "fallback": "t_null" },
+        "switch": { "param": "fencing_type", "fallback": "t_region_soil" },
         "cases": {
           "t_region_shrub_decorative": "f_null",
           "t_region_soil": "f_hedge_short",
           "t_region_groundcover_urban": "f_null",
           "t_chainfence": "f_null",
           "t_fence": "f_null",
           "t_privacy_fence": "f_null",
-          "t_drystone_wall_half": "f_null",
-          "t_null": "f_null"
+          "t_drystone_wall_half": "f_null"
         }
       },
       "%": {
-        "switch": { "param": "shrubbery_type", "fallback": "t_null" },
+        "switch": { "param": "shrubbery_type", "fallback": "t_region_soil" },
         "cases": {
           "t_region_shrub_decorative": "f_null",
           "t_region_soil": "f_hedge_short",
           "t_region_groundcover_urban": "f_null",
-          "t_region_groundcover": "f_region_flower_decorative",
-          "t_null": "f_null"
+          "t_region_groundcover": "f_region_flower_decorative"
         }
       }
     }

data/json/mapgen_palettes/park_palette.json

@@ -125,13 +125,12 @@
     },
     "furniture": {
       "Y": {
-        "switch": { "param": "walkway", "fallback": "t_null" },
+        "switch": { "param": "walkway", "fallback": "t_region_soil" },
         "cases": {
           "t_railroad_rubble": "f_null",
           "t_region_soil": "f_null",
           "t_region_groundcover_barren": "f_null",
-          "t_concrete": "f_street_light",
-          "t_null": "f_null"
+          "t_concrete": "f_street_light"
         }
       },
       "T": "f_table",