Support optional values during non-preview run

[andrzejressel]

Currently all null values from Pulumi are treated as Nothing which is correct during preview, but incorrect during non-preview.

Test case:

Index: examples/simple/src/lib.rs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/examples/simple/src/lib.rs b/examples/simple/src/lib.rs
--- a/examples/simple/src/lib.rs	(revision 00e265730b313e9dcd331ec220c49f5844766964)
+++ b/examples/simple/src/lib.rs	(date 1727119217354)
@@ -19,6 +19,9 @@
     // Tests number mapping
     let number = random_string.min_upper.map(|i| i * 2);
 
+    // Optional values are deserialized as None
+    let keepers = random_string.keepers.map(|map| format!("Keepers: {map:?}"));
+    
     let val1 = Output::new(&1);
     let val2 = Output::new(&"abc".to_string());
 
@@ -34,5 +37,6 @@
     add_export("number", &number);
     add_export("combined_string", &combined_string);
     add_export("combined_2_string", &combined_2_string);
+    add_export("keepers", &keepers);
     Ok(())
 }

keepers output is not shown