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/libs/utils/utilsicons.cpp

@@ -458,6 +458,10 @@ QIcon CodeModelIcon::iconForType(CodeModelIcon::Type type)
         }, Icon::Tint).icon());
         return icon;
     }
+    case Unknown: {
+        const static QIcon icon(Icons::EMPTY16.icon());
+        return icon;
+    }
     default:
         break;
     }