[chore] Newsdownloader: Fix more luacheck warnings (#8220)

plugins/newsdownloader.koplugin/epubdownloadbackend.lua

@@ -45,7 +45,7 @@ local function filter(text, element)
         "div#newsstorytext",
         "div.general",
         }
-    if element then
+    if element and element ~= "" then
         table.insert(selectors, 1, element)
     end
     for _, sel in ipairs(selectors) do

plugins/newsdownloader.koplugin/feed_view.lua

@@ -63,14 +63,10 @@ function FeedView:getItem(id, feed, edit_feed_callback, delete_feed_callback)
     end
 
     -- Collect this stuff for later, with the single view.
-    local download_full_article = feed.download_full_article == nil
-        or feed.download_full_article
-    local include_images = not never_download_images
-        and feed.include_images
-    local enable_filter = feed.enable_filter
-        or feed.enable_filter == nil
+    local download_full_article = feed.download_full_article ~= false
+    local include_images = feed.include_images ~= false
+    local enable_filter = feed.enable_filter ~= false
     local filter_element = feed.filter_element
-        or feed.filter_element == nil
 
     local vc = {
         {