handle nil value accessing CompletionItemKind (#690)

this prevents error messages from polluting the screen while using [ols](https://github.com/DanielGavin/ols) for autocompletion.

lua/cmp/entry.lua

@@ -218,7 +218,7 @@ entry.get_view = function(self, suggest_offset, entries_buf)
       view.kind.text = item.kind or ''
       view.kind.bytes = #view.kind.text
       view.kind.width = vim.fn.strdisplaywidth(view.kind.text)
-      view.kind.hl_group = 'CmpItemKind' .. types.lsp.CompletionItemKind[self:get_kind()]
+      view.kind.hl_group = 'CmpItemKind' .. (types.lsp.CompletionItemKind[self:get_kind()] or '')
       view.menu = {}
       view.menu.text = item.menu or ''
       view.menu.bytes = #view.menu.text