Use 1 = 0 as the clause for a :value => [] case

This case can be sent to the datastore as part of an invalid
branch for an OrOperation. This makes sure that IN () is not sent
to the database, since that's invalid syntax for SQL. The 1 = 0
is equivalent to IN () in this case.

lib/dm-core/adapters/data_objects_adapter.rb

@@ -617,6 +617,8 @@ def comparison_statement(comparison, qualify)
             end
           elsif @negated && comparison.slug == :in && !value.any?
             return [ '1 = 1' ]  # match everything
+          elsif comparison.slug == :in && !value.any?
+            return [ '1 = 0' ]  # match nothing
           end
 
           operator = comparison_operator(comparison)

lib/dm-core/spec/adapter_shared_spec.rb

@@ -181,6 +181,13 @@ class ::Heffalump
             Heffalump.all(:color.not => []).should == [ @red, @two, @five ]
           end
 
+          it 'should be able to search for objects in an empty list and another OR condition (match none on the empty list)' do
+            Heffalump.all(:conditions => DataMapper::Query::Conditions::Operation.new(
+                                           :or,
+                                           DataMapper::Query::Conditions::Comparison.new(:in, Heffalump.properties[:color], []),
+                                           DataMapper::Query::Conditions::Comparison.new(:in, Heffalump.properties[:num_spots], [5]))).should == [ @five ]
+          end
+
           it 'should be able to search for objects not included in an array of values' do
             Heffalump.all(:num_spots.not => [ 1, 3, 5, 7 ]).should be_include(@two)
           end