Add Buffer.isEmpty()

To be used by Buffer.isModified() for when the
buffer has no backing file

src/app/buffer.coffee

@@ -244,10 +244,12 @@ class Buffer
     if @file
       @memoryContentSignature != @diskContentSignature
     else
-      @lines.length > 1 or @lines[0]
+      not @isEmpty()
 
   isInConflict: -> @conflict
 
+  isEmpty: -> @lines.length is 1 and @lines[0].length is 0
+
   getAnchors: -> new Array(@anchors...)
 
   addAnchor: (options) ->