Java to Kotlin converter: generate "!!." instead of "?." for nullable values

#KT-3943 Fixed

j2k/tests/testData/ast/nullability/NullableMethodDotAccess.kt

@@ -4,6 +4,6 @@ class C() {
     }
 
     fun foo(): Int {
-        return getString(true)?.length()!!
+        return getString(true)!!.length()
     }
-}
+}

j2k/tests/testData/ast/nullability/NullableVariableDotAccess.kt

@@ -1,5 +1,5 @@
 fun foo(s: String?, b: Boolean): Int {
     if (s == null) System.out.println("null")
-    if (b) return s?.length()!!
+    if (b) return s!!.length()
     return 10
 }