fix using late-commands and reset-partition-only at the same time

examples/autoinstall/reset-only.yaml

@@ -9,3 +9,5 @@ storage:
     name: direct
     reset-partition: yes
     reset-partition-only: yes
+late-commands:
+  - ['true']

subiquity/server/controllers/cmdlist.py

@@ -129,7 +129,8 @@ class LateController(CmdListController):
 
     def env(self):
         env = super().env()
-        env['TARGET_MOUNT_POINT'] = self.app.base_model.target
+        if self.app.base_model.target is not None:
+            env['TARGET_MOUNT_POINT'] = self.app.base_model.target
         return env
 
     def start(self):