[NI] Report unsafe invoke properly for flexible types

`isNullable` check returns `true` for flexible types,
which leads to false positive unsafe invoke error, when
receiver for invoke's property is flexible.
^KT-30695 Fixed

compiler/testData/diagnostics/tests/resolve/invoke/kt30695_2.kt

@@ -1,3 +1,13 @@
+// FILE: JavaClass.java
+
+public class JavaClass {
+    public static Cls createFlexible() {
+        return new Cls();
+    }
+}
+
+// FILE: test.kt
+
 class MemberInvokeOwner {
     operator fun invoke() {}
 }
@@ -23,3 +33,11 @@ fun testNotNullableReceiver(notNullable: Cls) {
     notNullable.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
     notNullable<!UNNECESSARY_SAFE_CALL!>?.<!>extensionProperty()
 }
+
+fun testFlexibleReceiver() {
+    val flexible = JavaClass.createFlexible()
+    flexible.extensionProperty()
+    flexible?.<!UNSAFE_IMPLICIT_INVOKE_CALL!>extensionProperty<!>()
+    flexible.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
+    flexible?.<!UNSAFE_IMPLICIT_INVOKE_CALL!>nullableExtensionProperty<!>()
+}