Consider making .File on .Page a zero value instead of nil

[bep]

• It's now a method that returns an interface
• It's nil if no set
• The embedded .File is deprecated so you need to say .File.Filename instead of .Filename (you will get a deprecation warning for the latter)
• If you do .File.Filename on a Page without a File you will get a nilpointer (a nice error message pointing to the location in Go 1.12.1
• With the latest work on if/with we could make it a zero value work better and create less noise in the wild, but I tend to think that it would be better for people to know that the File isn't there (this is what I see in the wild; I think getting the nilpointer will help fix actual bugs in the templates around where people assumes that every Page is backed by a file)

I create this issue as a "thinking issue". What do you think, @moorereason ?

[bep]

I have thought a little about this, and I suspect the current solution will create more noise than we'd care for. So I will do this:

• Return a zero value when no file is set that is a proxy that prints a generic warning about wrapping the code in {{ with .File ...}}
• That way we avoid the bulk of the noise and site breakage, but help people fix what in most cases is a logical bug

/cc @onedrawingperday

[onedrawingperday]

This looks like a great plan!

Once you make a commit let me know and I will take it for a spin.

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