only prepare installer when installer method is defined

defaults/main.yml

@@ -1,3 +1,3 @@
+debian_installer_method: false
 debian_installer_directory: host_files/{{ inventory_hostname }}/debian-installer
 debian_installer_nonfree_firmware: false
-debian_installer_method: null

tasks/main.yml

@@ -8,7 +8,9 @@
     path: "{{debian_installer_directory}}"
 
 - import_tasks: preseed.yml
+
 - import_tasks: installer.yml
+  when: debian_installer_method
 
 - include: method/kexec.yml
   when: debian_installer_method == "kexec"