Fix annoying .File.Path on zero object. warning

[arunoruto]

When I was creating a post, I was getting this warning:

WARN  .File.Path on zero object. Wrap it in if or with: {{ with .File }}{{ .Path }}{{ end }}

So I did as suggested and replaced the .File.Path using a with statement in the list.html and single.html files. So far, the warning hasn't popped up again.
I am not sure why this warning comes up, but I hope this will mitigate weird behavior in the future.

I noticed this warning in two issues #602 and #490