Check the stat dictionary's size, not the variable's size

The ansible.builtin.stat module returns a dictionary where the
key to use is 'stat' in order to get the file stats. Add the
missing stat key. Failure to do this causes the task to fail
with size not being a key in the dictionary.

ref: https://docs.ansible.com/ansible/latest/collections/ansible/builtin/stat_module.html#ansible-collections-ansible-builtin-stat-module

roles/handle-constraints-url/tasks/main.yaml

@@ -23,4 +23,4 @@
       TOX_CONSTRAINTS_FILE: "{{ constraints_file.path }}"
       # Backward compatibility, to be removed
       UPPER_CONSTRAINTS_FILE: "{{ constraints_file.path }}"
-  when: constraints_stat.size > 0
+  when: constraints_file is defined and constraints_stat.size > 0