LanguageClient: Use empty icon for unknown completion item kind

...instead of a null icon.

Without this, a completion item gets no icon and looks misaligned
between items with icons. For example, for the completion item "size_t"
(a typedef) clangd reports a completion kind of "Reference", for which
we do not have a dedicated icon.

Change-Id: Ibba658d6bfb958bb34747e3f57514af49f5427b0
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/languageclient/languageclientcompletionassist.cpp

@@ -140,8 +140,7 @@ QIcon LanguageClientCompletionItem::icon() const
     case CompletionItemKind::Snippet: icon = QIcon(":/texteditor/images/snippet.png"); break;
     case CompletionItemKind::EnumMember: icon = iconForType(Enumerator); break;
     case CompletionItemKind::Struct: icon = iconForType(Struct); break;
-    default:
-        break;
+    default: icon = iconForType(Unknown); break;
     }
     return icon;
 }