Fix compilation error

Summary:
We are getting a compilation error on master:
```
File "command/test/baseConfigurationTest.ml", line 38, characters 22-29:
38 |     system_platform = "linux";
                           ^^^^^^^
Error: This expression has type string but an expression was expected of type
         string option
```
Let's fix it.

Reviewed By: dkgi, Healdb

Differential Revision: D64109699

fbshipit-source-id: 94d36c2ce86cf01d877e1810359d5c02cf967e9d

source/command/test/baseConfigurationTest.ml

@@ -35,7 +35,7 @@ let dummy_base_configuration =
     debug = false;
     enable_type_comments = true;
     python_version = Configuration.PythonVersion.create ();
-    system_platform = "linux";
+    system_platform = None;
     parallel = false;
     number_of_workers = 1;
     long_lived_workers = None;