Fix nullable properties of TypeVariable types

We were forgetting to apply the property's nullability to the resolved type.

Fixes #521

kotlin-codegen/integration-test/src/test/kotlin/com/squareup/moshi/GeneratedAdaptersTest.kt

@@ -222,14 +222,16 @@ class GeneratedAdaptersTest {
         listOf("foo", null, "bar"),
         setOf("foo", null, "bar"),
         mapOf("foo" to "bar", "baz" to null),
-        null
+        null,
+        1
     )
 
     val noNullsTypeParams = NullableTypeParams<Int>(
         nullableTypeParams.nullableList,
         nullableTypeParams.nullableSet,
         nullableTypeParams.nullableMap.filterValues { it != null },
-        null
+        null,
+        1
     )
 
     val json = adapter.toJson(nullableTypeParams)
@@ -782,7 +784,8 @@ data class NullableTypeParams<T>(
     val nullableList: List<String?>,
     val nullableSet: Set<String?>,
     val nullableMap: Map<String, String?>,
-    val nullableT: T?
+    val nullableT: T?,
+    val nonNullT: T
 )
 
 typealias TypeAliasName = String