Only set the head where there are elements in the collection

This caused sparse nil's to be added to a collection sometimes
which can cause very strange bugs

lib/dm-core/collection.rb

@@ -216,7 +216,7 @@ def first(*args)
         collection
       elsif with_query || loaded?
         collection.to_a.first
-      else
+      elsif collection.any?
         head[0] = collection.to_a.first
       end
     end

spec/public/shared/finder_shared_spec.rb

@@ -732,6 +732,21 @@
       end
     end
 
+    describe 'on an empty collection' do
+      before :all do
+        @articles = @articles.all(:id => nil)
+        @return = @articles.first
+      end
+
+      it 'should still be an empty collection' do
+        @articles.should be_empty
+      end
+
+      it 'should return nil' do
+        @return.should be_nil
+      end
+    end
+
     describe 'with offset specified' do
       before :all do
         @return = @resource = @articles.first(:offset => 1)