fix: reading chunks from an empty (zero-length) grid-stored file

reading chunks from an empty file caused an endless loop

lib/mongo/gridfs/grid_io.rb

@@ -253,6 +253,7 @@ module Mongo
       return read_all unless block_given?
       while chunk = read(chunk_size) 
         yield chunk
+        break if chunk.empty?
       end
       self
     end