INT-4067: Cover empty file case in FileSplitter

JIRA: https://jira.spring.io/browse/INT-4067

When `FileSplitter` is configured with `markers = true` and file is empty, an `iterator` for file throws `IOException: Stream closed`,
because we close the `buffer` just after the first `readLine()` attempt, but still return `true` from the first `hasNext()` call
where the `this.sof` and `this.eof` are `true` for markers.

Add logic to mark internal splitter `iterator` as `done` where we don't have content and still in `sof` state.

spring-integration-file/src/main/java/org/springframework/integration/file/splitter/FileSplitter.java

@@ -235,6 +235,9 @@ public class FileSplitter extends AbstractMessageSplitter {
 					if (!ready) {
 						if (this.markers) {
 							this.eof = true;
+							if (this.sof) {
+								this.done = true;
+							}
 						}
 						bufferedReader.close();
 					}