Fix isNullableType() to always consider flexible types, even if they contain type parameters

compiler/testData/codegen/box/javaInterop/notNullAssertions/mapPut.kt

@@ -0,0 +1,11 @@
+import java.util.HashMap
+
+fun <K: Any, V: Any> foo(k: K, v: V) {
+    val map = HashMap<K, V>()
+    val old = map.put(k, v)
+}
+
+fun box(): String {
+    foo("", "")
+    return "OK"
+}