bugfix: open empty files

Why this matters:
- Right now there tends to be no way (in this app and for most of my
  LÖVE apps) to open a new file.
- The obvious workaround without access to a terminal is to create a new
  file and drop it on the app window.

So it's important for my apps, when they edit a single file, to handle
an empty input file.

It's also important to remember that json.decode will raise an error on
an empty string. I don't think my other apps (except forks of snap.love)
have an issue with this. They tend to open existing files, create files
before opening them (mastodon-unfurl) or not have purely JSON-based file
formats.

0012-on.initialize

@@ -6,8 +6,6 @@ on.initialize = function(arg)
 	if Settings[Filename] then
 		Viewport = Settings[Filename].viewport
 	end
-	if file_exists(Filename) then
-		load_graph_from_disk()
-	end
+	load_graph_from_disk()
 	A()
-end
+end

0063-load_graph_from_disk

@@ -1,7 +1,10 @@
 load_graph_from_disk = function()
 	local f = App.open_for_reading(Filename)
-	local data = json.decode(f:read('*a'))
+	if f == nil then return end
+	local contents = f:read('*a')
 	f:close()
+	if contents == '' then return end
+	local data = json.decode(contents)
 	Nodes = data.nodes or {}
 	First_available_id = data.next
 end

0064-on.file_drop

@@ -5,8 +5,6 @@ on.file_drop = function(file)
 	end
 	Filename = file:getFilename()
 	love.window.setTitle('snap.love - '..Filename)
-	if file_exists(Filename) then
-		load_graph_from_disk()
-	end
+	load_graph_from_disk()
 	A()
-end
+end