nixd/test/completion: precommit pkgs failing test (#550)

Dereferencing empty vector and thus crash.

nixd/tools/nixd/test/completion/pkgs.md

@@ -0,0 +1,78 @@
+# RUN: nixd --lit-test \
+# RUN: --nixpkgs-expr="{ lib.hello.meta.description = \"Very Nice\";  }" \
+# RUN: < %s | FileCheck %s
+# XFAIL: *
+
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "method":"textDocument/didOpen",
+   "params":{
+      "textDocument":{
+         "uri":"file:///completion.nix",
+         "languageId":"nix",
+         "version":1,
+         "text":"pkgs"
+      }
+   }
+}
+```
+
+```json
+{
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "textDocument/completion",
+    "params": {
+        "textDocument": {
+            "uri": "file:///completion.nix"
+        },
+        "position": {
+            "line": 0,
+            "character": 3
+        },
+        "context": {
+            "triggerKind": 1
+        }
+    }
+}
+```
+
+```
+     CHECK:    "isIncomplete": false,
+CHECK-NEXT:    "items": [
+CHECK-NEXT:      {
+CHECK-NEXT:        "data": "{\"Prefix\":\"hel\",\"Scope\":[\"lib\"]}",
+CHECK-NEXT:        "kind": 5,
+CHECK-NEXT:        "label": "hello",
+CHECK-NEXT:        "score": 0
+CHECK-NEXT:      }
+CHECK-NEXT:    ]
+```
+
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```