Improve empty-string-compare rule (#1480)

- rewrite matching to avoid line processing and trigger only on when
  blocks, highly reducing the chance of false-positives
- add additional pass test to avoid future regression
- refactor rule to use embedded tests

Fixes: #1232

src/ansiblelint/rules/ComparisonToEmptyStringRule.py

@@ -2,9 +2,11 @@
 # Copyright (c) 2018, Ansible Project
 
 import re
-from typing import Union
+import sys
+from typing import Any, Dict, Union
 
 from ansiblelint.rules import AnsibleLintRule
+from ansiblelint.utils import nested_items
 
 
 class ComparisonToEmptyStringRule(AnsibleLintRule):
@@ -20,5 +22,64 @@ class ComparisonToEmptyStringRule(AnsibleLintRule):
 
     empty_string_compare = re.compile("[=!]= ?(\"{2}|'{2})")
 
-    def match(self, line: str) -> Union[bool, str]:
-        return bool(self.empty_string_compare.search(line))
+    def matchtask(self, task: Dict[str, Any]) -> Union[bool, str]:
+        for k, v, _ in nested_items(task):
+            if k == 'when':
+                if isinstance(v, str):
+                    if self.empty_string_compare.search(v):
+                        return True
+                elif isinstance(v, bool):
+                    pass
+                else:
+                    for item in v:
+                        if isinstance(item, str) and self.empty_string_compare.search(
+                            item
+                        ):
+                            return True
+
+        return False
+
+
+# testing code to be loaded only with pytest or when executed the rule file
+if "pytest" in sys.modules:
+
+    import pytest
+
+    SUCCESS_PLAY = '''
+- hosts: all
+  tasks:
+    - name: shut down
+      shell: |
+        /sbin/shutdown -t now
+        echo $var == ""
+      when: ansible_os_family
+'''
+
+    FAIL_PLAY = '''
+- hosts: all
+  tasks:
+  - name: shut down
+    command: /sbin/shutdown -t now
+    when: ansible_os_family == ""
+  - name: shut down
+    command: /sbin/shutdown -t now
+    when: ansible_os_family !=""
+'''
+
+    @pytest.mark.parametrize(
+        'rule_runner', (ComparisonToEmptyStringRule,), indirect=['rule_runner']
+    )
+    def test_rule_empty_string_compare_fail(rule_runner: Any) -> None:
+        """Test rule matches."""
+        results = rule_runner.run_playbook(FAIL_PLAY)
+        assert len(results) == 2
+        for result in results:
+            assert result.message == ComparisonToEmptyStringRule.shortdesc
+
+    @pytest.mark.parametrize(
+        'rule_runner', (ComparisonToEmptyStringRule,), indirect=['rule_runner']
+    )
+    def test_rule_empty_string_compare_pass(rule_runner: Any) -> None:
+        """Test rule matches."""
+        results = rule_runner.run_playbook(SUCCESS_PLAY)
+        assert len(results) == 0, results