Refactor nil checking for video_duration

This commit short-circuits the following check added in the previous commit (d8fcff5) [1]:

```
    if video_duration and video_duration ~= nil then
      video_duration = tonumber(video_duration)
    else
      video_duration = 0
    end
```

[1] gsbabil@d8fcff5

Author: gsbabil

scripts/SimpleHistory.lua

@@ -2115,12 +2115,8 @@ function history_resume_option()
 		if video_time > 0 then return end
 		local logged_time = 0
 		local percentage = 0
-		local video_duration = mp.get_property_number('duration')
-		if video_duration and video_duration ~= nil then
-			video_duration = tonumber(video_duration)
-		else
-			video_duration = 0
-		end	
+		local video_duration = mp.get_property_number('duration') or 0
+
 		list_contents = read_log_table()
 		if not list_contents or not list_contents[1] then return end
 		for i = #list_contents, 1, -1 do