Fix 'return null' statements

Author: szarnyasg

src/main/java/ldbc/snb/datagen/serializer/empty/EmptyDynamicActivitySerializer.java

@@ -54,7 +54,7 @@ public class EmptyDynamicActivitySerializer extends DynamicActivitySerializer {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override

src/main/java/ldbc/snb/datagen/serializer/empty/EmptyDynamicPersonSerializer.java

@@ -52,7 +52,7 @@ public class EmptyDynamicPersonSerializer extends DynamicPersonSerializer {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override

src/main/java/ldbc/snb/datagen/serializer/empty/EmptyStaticSerializer.java

@@ -52,7 +52,7 @@ public class EmptyStaticSerializer extends StaticSerializer {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override

src/main/java/ldbc/snb/datagen/serializer/graphalytics/CsvDynamicPersonSerializer.java

@@ -49,6 +49,7 @@ Linked Data Benchmark Council (http://www.ldbcouncil.org)
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 public class CsvDynamicPersonSerializer extends DynamicPersonSerializer<HdfsCsvWriter> implements CsvSerializer {
@@ -71,7 +72,7 @@ public String toString() {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override

src/main/java/ldbc/snb/datagen/serializer/graphalytics/CsvDynamicPersonSerializerExtended.java

@@ -50,6 +50,7 @@ Linked Data Benchmark Council (http://www.ldbcouncil.org)
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 public class CsvDynamicPersonSerializerExtended extends DynamicPersonSerializer<HdfsCsvWriter> implements CsvSerializer {
@@ -74,7 +75,7 @@ public String toString() {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override

src/main/java/ldbc/snb/datagen/serializer/graphalytics/pgx/CsvDynamicPersonSerializer.java

@@ -49,6 +49,7 @@ Linked Data Benchmark Council (http://www.ldbcouncil.org)
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 public class CsvDynamicPersonSerializer extends DynamicPersonSerializer<HdfsCsvWriter> implements CsvSerializer {
@@ -71,7 +72,7 @@ public String toString() {
 
     @Override
     public List<FileName> getFileNames() {
-        return null;
+        return Collections.emptyList();
     }
 
     @Override