empty string compare rule docs

src/ansiblelint/rules/empty_string_compare.md

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+# empty-string-compare
+
+This rule checks for empty string comparison in playbooks.
+To ensure code clarity you should avoid using empty strings in conditional statements with the `when` clause.
+
+- Use `when: var|length > 0` instead of `when: var != ""`.
+- Use `when: var|length == 0` instead of `when: var == ""`.
+
+This is an opt-in rule.
+You must enable it in your Ansible-lint configuration as follows:
+
+```yaml
+enable_list:
+  - empty-string-compare
+```
+
+## Problematic Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: all
+  tasks:
+    - name: Shut down
+      ansible.builtin.command: /sbin/shutdown -t now
+      when: ansible_os_family == "" # <- Compares with an empty string.
+    - name: Shut down
+      ansible.builtin.command: /sbin/shutdown -t now
+      when: ansible_os_family !="" # <- Compares with an empty string.
+```
+
+## Correct Code
+
+```yaml
+---
+- name: Example playbook
+  hosts: all
+  tasks:
+    - name: Shut down
+      ansible.builtin.shell: |
+        /sbin/shutdown -t now
+        echo $var ==
+      when: ansible_os_family
+```