GH-3395: Fix XML expression default for ARFOGateway

Resolves https://github.com/spring-projects/spring-integration/issues/3395

Outbound remote file gateway parser requires `expression` even though some
commands don't need or use it.

* Propagate the empty string for `expression` attribute
* Fix default (`payload`) expression logic in the `AbstractRemoteFileOutboundGateway`

**Cherry-pick to `5.3.x`**

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/config/SftpOutboundGatewayParserTests-context.xml

@@ -113,4 +113,26 @@
 
 	<int:channel id="outbound"/>
 
+	<int-sftp:outbound-gateway id="noExpressionLS"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		command="ls" />
+
+	<int-sftp:outbound-gateway id="noExpressionPUT"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		command="put" />
+
+	<int-sftp:outbound-gateway id="noExpressionGET"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		local-directory="."
+		command="get" />
+
 </beans>

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/config/SftpOutboundGatewayParserTests.java

@@ -69,6 +69,15 @@ public class SftpOutboundGatewayParserTests {
 	@Autowired
 	AbstractEndpoint advised;
 
+	@Autowired
+	AbstractEndpoint noExpressionLS;
+
+	@Autowired
+	AbstractEndpoint noExpressionPUT;
+
+	@Autowired
+	AbstractEndpoint noExpressionGET;
+
 	@Autowired
 	FileNameGenerator generator;
 
@@ -165,6 +174,14 @@ public class SftpOutboundGatewayParserTests {
 		assertThat(adviceCalled).isEqualTo(1);
 	}
 
+	@Test
+	void noExpression() {
+		assertThat(TestUtils.getPropertyValue(this.noExpressionLS, "handler.fileNameProcessor")).isNull();
+		assertThat(TestUtils.getPropertyValue(this.noExpressionPUT, "handler.fileNameProcessor")).isNull();
+		assertThat(TestUtils.getPropertyValue(this.noExpressionGET,
+				"handler.fileNameProcessor.expression.expression")).isEqualTo("payload");
+	}
+
 	public static class FooAdvice extends AbstractRequestHandlerAdvice {
 
 		@Override