Do not generate DefaultImpls if empty

compiler/testData/codegen/bytecodeListing/defaultImpls.kt

@@ -0,0 +1,17 @@
+interface A
+
+interface B {
+    fun foo()
+}
+
+interface C {
+    val bar: Int
+}
+
+interface D {
+    fun baz() = 5
+}
+
+interface E {
+    class InsideE
+}