luci-lib-base: ensure that luci.http can be required standalone

Various existing Lua software is requiring the `luci.http` library for
URL encoding/decoding tasks so ensure that it can be loaded in a stand
alone manner even if the emulated Lua runtime environment is not
available.

Fixes: cea2c3578e ("luci-lib-base: forward luci.http.context.request.message to ucode")
Ref: https://forum.openwrt.org/t/x/141817
Ref: https://github.com/openwrt/luci/pull/5976#issuecomment-1296220586
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

libs/luci-lib-base/luasrc/http.lua

@@ -140,6 +140,6 @@ context = {
 		content        = function(self, ...) return content(...)        end;
 		getcookie      = function(self, ...) return getcookie(...)      end;
 		setfilehandler = function(self, ...) return setfilehandler(...) end;
-		message        = L.http.message
+		message        = L and L.http.message
 	}
 }