return nil instead of '' for mocks as well

lib/fog/aws/requests/ec2/allocate_address.rb

@@ -27,7 +27,7 @@ module Fog
             response.status = 200
             public_ip = Fog::AWS::Mock.ip_address
             data ={
-              'instanceId' => '',
+              'instanceId' => nil,
               'publicIp'   => public_ip
             }
             @data[:addresses][public_ip] = data

lib/fog/aws/requests/ec2/create_snapshot.rb

@@ -35,7 +35,7 @@ module Fog
             response.status = 200
             snapshot_id = Fog::AWS::Mock.snapshot_id
             data = {
-              'progress'    => '',
+              'progress'    => nil,
               'snapshotId'  => snapshot_id,
               'startTime'   => Time.now,
               'status'      => 'pending',

lib/fog/aws/requests/ec2/create_volume.rb

@@ -43,7 +43,7 @@ module Fog
               'attachmentSet'     => [],
               'createTime'        => Time.now,
               'size'              => size,
-              'snapshotId'        => snapshot_id || '',
+              'snapshotId'        => snapshot_id,
               'status'            => 'creating',
               'volumeId'          => volume_id
             }

lib/fog/aws/requests/ec2/disassociate_address.rb

@@ -29,7 +29,7 @@ module Fog
           response = Excon::Response.new
           response.status = 200
           if address = @data[:addresses][public_ip]
-            address['instanceId'] = ''
+            address['instanceId'] = nil
             response.status = 200
             response.body = {
               'requestId' => Fog::AWS::Mock.request_id,

lib/fog/aws/requests/ec2/run_instances.rb

@@ -107,7 +107,7 @@ module Fog
             instance = {
               'amiLaunchIndex'      => i,
               'blockDeviceMapping'  => [],
-              'dnsName'             => '',
+              'dnsName'             => nil,
               'imageId'             => image_id,
               'instanceId'          => instance_id,
               'instanceState'       => { 'code' => 0, 'name' => 'pending' },
@@ -117,17 +117,17 @@ module Fog
               'launchTime'          => Time.now,
               'monitoring'          => { 'state' => options['Monitoring.Enabled'] || false },
               'placement'           => { 'availabilityZone' => options['Placement.AvailabilityZone'] || Fog::AWS::Mock.availability_zone },
-              'privateDnsName'      => '',
+              'privateDnsName'      => nil,
               'productCodes'        => [],
               'ramdiskId'           => options['RamdiskId'] || Fog::AWS::Mock.ramdisk_id,
-              'reason'              => '',
+              'reason'              => nil,
               'rootDeviceType'      => 'instance-store'
             }
             instances_set << instance
             @data[:instances][instance_id] = instance.merge({
               'groupSet'            => group_set,
               'ownerId'             => @owner_id,
-              'privateIpAddress'    => '',
+              'privateIpAddress'    => nil,
               'reservationId'       => reservation_id,
             })
           end