fix: attempt to index ? (a nil value)

When a new player logged into the game the game crashed and showed
the above error as the message. The reason for this is due to not
having the default learning course (which is "" - nothing) in the
courses table
Fixed that error

game.lua

@@ -1007,7 +1007,9 @@ function setUI(name)
     ui.addTextArea(4, "<a href='event:shop'><b><font color='#000000' size='15'>Shop</font></b></a>", name, 100, 230, 50, 40, nil, nil, 0, false)
     --school button
     ui.addTextArea(5, "<a href='event:courses'><font size='15'><b>Enter</b></font></a>", name, 600, 270, 60, 20, nil, nil, 0, false)
-    ui.addTextArea(3000, "<p align='center'><b>Lessons left: " .. p:getLearningProgress() .." / " .. courses[p.learning].lessons .. "</b></p>", name, 480, 180, 300, 20, nil, nil, 0, false)
+    if not p.learning == "" then
+        ui.addTextArea(3000, "<p align='center'><b>Lessons left: " .. p:getLearningProgress() .." / " .. courses[p.learning].lessons .. "</b></p>", name, 480, 180, 300, 20, nil, nil, 0, false)
+    end
     --jobs button
     tfm.exec.addImage("16f88c66ed1.png", ":10", 610, 25) -- Job search image by Freepik in 'Flaticon.com'
     ui.addTextArea(6, "<a href='event:jobs'>\t\n\t\n\t\n</a>", name, 610, 25, 30, 30, nil, nil, 0, true)