[fix] Always initiate empty context table (#6874)

In case of no environment language. I suppose that doesn't tend to happen much in the wild since I added the offending code early in 2020. ;-)

Fixes <https://github.com/koreader/koreader/issues/6873>.

frontend/gettext.lua

@@ -23,6 +23,7 @@ local isAndroid, android = pcall(require, "android")
 local logger = require("logger")
 
 local GetText = {
+    context = {},
     translation = {},
     current_lang = "C",
     dirname = "l10n",