fix(extension/#3787): Fix completion item parsing resiliency (#3800)

Fix #3787 

__Issue:__ Completion items were coming with `null` value in the JSON, for example: 
```
{
...
'c': null,
...
}
```
(Note that the completion item JSON format uses a condensed field name to minimize data over the wire, see: https://github.com/onivim/vscode-exthost/blob/fc0cb0ba738b856da6ab36d0efe0aa3446959a98/src/vs/workbench/api/common/extHost.protocol.ts#L1383)

This was causing parsing to fail.

__Defect:__ Our parser was expecting the `detail` field (along with others) to either be present as a string or not present at all. The parser wasn't resilient in the case where the field was there, but `null`.

__Fix:__ Instead of using `field.optional`, use `field.withDefault` along with a `None` default value.

__Future work:__
Potentially `field.optional` should just work like this out-of-the-box...

__Todo:__
- [x] Changes entry

CHANGES_CURRENT.md

@@ -6,6 +6,7 @@
 ### Bug Fixes
 
 - #3791 - SCM: Fix 'Git: Checkout to' command (fixes #2525)
+- #3800 - Completion: Fix completion item parsing resiliency (fixes #3787)
 
 ### Performance
 

src/Exthost/SuggestItem.re

@@ -128,13 +128,25 @@ let sortText = ({sortText, label, _}) => {
   };
 };
 
+module SuggestItemLabel = {
+  type t = string;
+
+  let decode =
+    Json.Decode.(
+      one_of([
+        ("string", string),
+        ("record", obj(({field, _}) => {field.required("label", string)})),
+      ])
+    );
+};
+
 module Dto = {
   let decode = (~defaultRange) => {
     Json.Decode.(
       obj(({field, _}) => {
         // These fields come from the `ISuggestDataDtoField` definition:
         // https://github.com/onivim/vscode-exthost/blob/50bef147f7bbd250015361a4e3cad3305f65bc27/src/vs/workbench/api/common/extHost.protocol.ts#L1089
-        let label = field.required("a", string);
+        let label = field.required("a", SuggestItemLabel.decode);
 
         let kind =
           field.withDefault(
@@ -147,11 +159,12 @@ module Dto = {
                ),
           );
 
-        let detail = field.optional("c", string);
-        let documentation = field.optional("d", MarkdownString.decode);
-        let sortText = field.optional("e", string);
-        let filterText = field.optional("f", string);
-        let insertText = field.optional("h", string);
+        let detail = field.withDefault("c", None, nullable(string));
+        let documentation =
+          field.withDefault("d", None, nullable(MarkdownString.decode));
+        let sortText = field.withDefault("e", None, nullable(string));
+        let filterText = field.withDefault("f", None, nullable(string));
+        let insertText = field.withDefault("h", None, nullable(string));
         let insertTextRules =
           field.withDefault(
             "i",