Add a test for handling missing detail keys for LSP completion

test/completion/test_lsp_completion_parsing.vader

@@ -429,3 +429,23 @@ Execute(Should handle Python completion results correctly):
   \     ]
   \   }
   \ })
+
+Execute(Should handle missing detail keys):
+  AssertEqual
+  \ [
+  \   {'word': 'x', 'menu': '', 'info': 'y', 'kind': 'f', 'icase': 1},
+  \ ],
+  \ ale#completion#ParseLSPCompletions({
+  \   'jsonrpc': '2.0',
+  \   'id': 6,
+  \   'result': {
+  \     'isIncomplete': v:false,
+  \     'items': [
+  \       {
+  \         'label': 'x',
+  \         'kind': 3,
+  \         'documentation': 'y',
+  \       },
+  \     ]
+  \   }
+  \ })