provider/aws: ignore providers with Meta nil

We changed the way validation works for providers so that they aren't
always configured if they have computed attributes. The result is that
sometimes the Configure won't be called, hence Meta is nil

builtin/providers/aws/resource_aws_instance_test.go

@@ -579,6 +579,11 @@ func testAccCheckInstanceExistsWithProviders(n string, i *ec2.Instance, provider
 			return fmt.Errorf("No ID is set")
 		}
 		for _, provider := range *providers {
+			// Ignore if Meta is empty, this can happen for validation providers
+			if provider.Meta() == nil {
+				continue
+			}
+
 			conn := provider.Meta().(*AWSClient).ec2conn
 			resp, err := conn.DescribeInstances(&ec2.DescribeInstancesInput{
 				InstanceIDs: []*string{aws.String(rs.Primary.ID)},