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

reading chunks from an empty file caused an endless loop

Author: Daniël van de Burgt

lib/mongo/gridfs/grid_io.rb

@@ -251,8 +251,9 @@ def close
     # @return [Mongo::GridIO] self
     def each
       return read_all unless block_given?
-      while chunk = read(chunk_size)
+      while chunk = read(chunk_size) 
         yield chunk
+        break if chunk.empty?
       end
       self
     end