Handle removing sessions that don't exist

Resolves #21

src/main/java/org/springframework/session/data/mongo/MongoOperationsSessionRepository.java

@@ -23,11 +23,11 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 import javax.annotation.PostConstruct;
 
 import org.bson.Document;
-
 import org.springframework.data.mongodb.core.MongoOperations;
 import org.springframework.data.mongodb.core.index.IndexOperations;
 import org.springframework.data.mongodb.core.query.Query;
@@ -141,7 +141,8 @@ public class MongoOperationsSessionRepository
 
 	@Override
 	public void deleteById(String id) {
-		this.mongoOperations.remove(findSession(id), this.collectionName);
+		Optional.ofNullable(findSession(id))
+			.ifPresent(document -> this.mongoOperations.remove(document, this.collectionName));
 	}
 
 	@PostConstruct

src/test/java/org/springframework/session/data/mongo/integration/AbstractMongoRepositoryITest.java

@@ -137,6 +137,11 @@ abstract public class AbstractMongoRepositoryITest extends AbstractITest {
 		assertThat(findByPrincipalName.keySet()).doesNotContain(toSave.getId());
 	}
 
+	@Test
+	public void nonExistentSessionShouldNotBreakMongo() {
+		this.repository.deleteById("doesn't exist");
+	}
+
 	@Test
 	public void findByPrincipalNameNoPrincipalNameChange() throws Exception {