fix: make sure not to include unecessary ENV variable

This is a fix to an unfortunate bug introduced by #64 because we had a
default value set to `None` on the
`clever_haskell_entry_point`. Ansible considers `None` as a defined
value so the `is defined` condition doesn't match our need

defaults/main.yml

@@ -6,7 +6,7 @@ clever_app_root: "{{ app_root | default(playbook_dir + '/..') }}"
 clever_app_confdir: "{{ clever_app_root }}/.clever_cloud"
 clever_login_file: "{{ clever_app_confdir }}/login"
 
-clever_haskell_entry_point: "{{ clever_entry_point | default(None) }}"
+clever_haskell_entry_point: "{{ clever_entry_point | default(omit) }}"
 clever_haskell_env: {}
 clever_env: {}