MM-56780: Ensure p.Integration is not nil (#26301)

* Ensure p.Integration is not nil

A Sentry crash report showed that the check
    p.Integration.URL != input.integration.URL
was dereferencing a nil pointer. At that point in the code,
input.Integration is known to be non-nil, but we still need to check
whether the original, p.Integration, is.

Crash report: https://mattermost-mr.sentry.io/issues/4918263046/events/5738b67edcee4c9c883d40f7d26563a6

* Test nil original integration and both nil

server/public/model/integration_action_test.go

@@ -144,7 +144,7 @@ func TestPostActionIntegrationEquals(t *testing.T) {
 		require.False(t, pa1.Equals(pa2))
 	})
 
-	t.Run("nil check", func(t *testing.T) {
+	t.Run("nil check in input integration", func(t *testing.T) {
 		pa1 := &PostAction{
 			Integration: &PostActionIntegration{},
 		}
@@ -155,4 +155,28 @@ func TestPostActionIntegrationEquals(t *testing.T) {
 
 		require.False(t, pa1.Equals(pa2))
 	})
+
+	t.Run("nil check in original integration", func(t *testing.T) {
+		pa1 := &PostAction{
+			Integration: nil,
+		}
+
+		pa2 := &PostAction{
+			Integration: &PostActionIntegration{},
+		}
+
+		require.False(t, pa1.Equals(pa2))
+	})
+
+	t.Run("both nil", func(t *testing.T) {
+		pa1 := &PostAction{
+			Integration: nil,
+		}
+
+		pa2 := &PostAction{
+			Integration: nil,
+		}
+
+		require.True(t, pa1.Equals(pa2))
+	})
 }