Prohibit `self.assertRaisesMessage` with empty string

[GitRon]

User Story

As a Django developer,
I want the linter boa restrictor to flag any usage of self.assertRaisesMessage with an empty string as the expected message,
so that tests enforce meaningful error messages and avoid false positives.

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Acceptance Criteria

1. Detection

   • The linter must detect calls to self.assertRaisesMessage where the second argument is an empty string ("").
   • Example (should be flagged):

     self.assertRaisesMessage(ValueError, "")

2. Non-Detection (Valid Cases)

   • Calls where the second argument is a non-empty string must not be flagged.

     self.assertRaisesMessage(ValueError, "Invalid input")

   • Calls to other assert* methods (e.g., self.assertRaises, self.assertEqual) must not be flagged.
   • Calls to assertRaisesMessage outside of Django’s TestCase context may still be flagged (depending on implementation scope), but only empty strings should trigger the rule.

3. Error Message

   • The linter should produce a clear error message such as:

     boa restrictor: Do not use assertRaisesMessage with an empty string as the expected message.
     

4. Configurability

   • The rule must be part of the Django-only ruleset and enabled only when Django checks are active.

5. Fix Guidance

   • Documentation or autofix (if supported) should suggest replacing the empty string with either a meaningful message or using self.assertRaises if message matching is unnecessary.