MM-38497: Fix Sentry crash in PostAction.Equals (#18571)

* MM-38497: Fix Sentry crash in PostAction.Equals

We check for nil pointer before moving ahead.

https://mattermost.atlassian.net/browse/MM-38497

```release-note
NONE
```

* Fix lint errors

```release-note
NONE
```

* Update model/integration_action.go

Co-authored-by: Claudio Costa <cstcld91@gmail.com>

Co-authored-by: Claudio Costa <cstcld91@gmail.com>

model/integration_action.go

@@ -115,6 +115,14 @@ func (p *PostAction) Equals(input *PostAction) bool {
 	}
 
 	// Compare PostActionIntegration
+
+	// If input is nil, then return true if original is also nil.
+	// Else return false.
+	if input.Integration == nil {
+		return p.Integration == nil
+	}
+
+	// Both are unequal and not nil.
 	if p.Integration.URL != input.Integration.URL {
 		return false
 	}

model/integration_action_test.go

@@ -142,4 +142,16 @@ func TestPostActionIntegrationEquals(t *testing.T) {
 		}
 		require.False(t, pa1.Equals(pa2))
 	})
+
+	t.Run("nil check", func(t *testing.T) {
+		pa1 := &PostAction{
+			Integration: &PostActionIntegration{},
+		}
+
+		pa2 := &PostAction{
+			Integration: nil,
+		}
+
+		require.False(t, pa1.Equals(pa2))
+	})
 }