Fix default value for non-final module type (#1392)

This fixes an issue where the `module` type produces the wrong default
value.

Closes #1391

pkl-core/src/test/files/LanguageSnippetTests/input/types/moduleType2.pkl

@@ -0,0 +1,20 @@
+open module Foo
+
+prop: String
+
+myself: module?
+
+class Bar extends module {
+  prop2: Int
+}
+
+output {
+  value = new Bar {
+    prop = "hi"
+    prop2 = 15
+    myself {
+      prop = "hi again"
+      prop2 = 15
+    }
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/types/moduleType2.pcf

@@ -0,0 +1,7 @@
+prop = "hi"
+myself {
+  prop = "hi again"
+  myself = null
+  prop2 = 15
+}
+prop2 = 15