[FE] Don't throw assertion in OverrideResolver if directOverridden is empty

Those descriptors may be empty in case user made a mistake and tried to
  delegate implementation of abstract class instead of interface (and
  we don't add functions from abstract class to overriden descriptors
  of fake overrides in case of delegation by)

#KT-40510 Fixed

compiler/testData/diagnostics/tests/delegation/kt40510.fir.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-40510
+
+// FILE: foo/A.java
+package foo;
+
+public abstract class A {
+    // package-private
+    abstract void foo();
+}
+
+// FILE: main.kt
+import foo.A
+
+class DelegatedA(val a: A) : A by a