AbstractDiagnosticBasedMigrationInspection: fix case with nullable custom factory

#KT-36479 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/migration/AbstractDiagnosticBasedMigrationInspection.kt

@@ -48,11 +48,10 @@ abstract class AbstractDiagnosticBasedMigrationInspection<T : KtElement>(
                         .singleOrNull()
                         ?: error("Must have one diagnostic")
 
-                    val intentionAction = (if (customIntentionFactory != null)
-                        customIntentionFactory.invoke(diagnostic)
+                    val intentionAction = if (customIntentionFactory != null)
+                        customIntentionFactory.invoke(diagnostic) ?: return
                     else
-                        actionsFactory.createActions(diagnostic).singleOrNull())
-                        ?: error("Must have one fix")
+                        actionsFactory.createActions(diagnostic).ifEmpty { return }.singleOrNull() ?: error("Must have one fix")
 
                     val text = descriptionMessage() ?: DefaultErrorMessages.render(diagnostic)
                     problemDescriptors.add(

idea/testData/inspectionsLocal/warningOnMainUnusedParameterMigration/notApplicable.kt

@@ -0,0 +1,6 @@
+// LANGUAGE_VERSION: 1.4
+// PROBLEM: none
+
+var vgs: Int
+    get() = 0
+    set(value<caret>) {}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -13337,6 +13337,11 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
             KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("idea/testData/inspectionsLocal/warningOnMainUnusedParameterMigration"), Pattern.compile("^([\\w\\-_]+)\\.(kt|kts)$"), null, true);
         }
 
+        @TestMetadata("notApplicable.kt")
+        public void testNotApplicable() throws Exception {
+            runTest("idea/testData/inspectionsLocal/warningOnMainUnusedParameterMigration/notApplicable.kt");
+        }
+
         @TestMetadata("simple.kt")
         public void testSimple() throws Exception {
             runTest("idea/testData/inspectionsLocal/warningOnMainUnusedParameterMigration/simple.kt");