use nulls instead of empty strings

Author: dszakallas

src/main/scala/ldbc/snb/datagen/generator/serializers/ActivityOutputStream.scala

@@ -82,11 +82,11 @@ class ActivityOutputStream(
       post.getDeletionDate,
       post.isExplicitlyDeleted,
       post.getMessageId,
-      "",
+      None,
       post.getIpAddress.toString,
       Dictionaries.browsers.getName(post.getBrowserId),
-      Dictionaries.languages.getLanguageName(post.getLanguage),
-      post.getContent,
+      Some(Dictionaries.languages.getLanguageName(post.getLanguage)),
+      Some(post.getContent),
       post.getContent.length,
       post.getAuthor.getAccountId,
       post.getForumId,
@@ -136,11 +136,11 @@ class ActivityOutputStream(
       photo.getDeletionDate,
       photo.isExplicitlyDeleted,
       photo.getMessageId,
-      photo.getContent,
+      Some(photo.getContent),
       photo.getIpAddress.toString,
       Dictionaries.browsers.getName(photo.getBrowserId),
-      "",
-      "",
+      None,
+      None,
       0,
       photo.getAuthor.getAccountId,
       photo.getForumId,

src/main/scala/ldbc/snb/datagen/io/raw/csv/CsvRowEncoderInstances.scala

@@ -98,11 +98,11 @@ trait CsvRowEncoderInstances {
       t.deletionDate.toString,
       t.explicitlyDeleted.toString,
       t.id.toString,
-      t.imageFile,
+      t.imageFile.fold("")(_.toString),
       t.locationIP,
       t.browserUsed,
-      t.language,
-      t.content,
+      t.language.fold("")(_.toString),
+      t.content.fold("")(_.toString),
       t.length.toString,
       t.`CreatorPersonId`.toString,
       t.`ContainerForumId`.toString,

src/main/scala/ldbc/snb/datagen/model/raw.scala

@@ -105,11 +105,11 @@ object raw {
       deletionDate: Long,
       explicitlyDeleted: Boolean,
       id: Long,
-      imageFile: String,
+      imageFile: Option[String],
       locationIP: String,
       browserUsed: String,
-      language: String,
-      content: String,
+      language: Option[String],
+      content: Option[String],
       length: Int,
       `CreatorPersonId`: Long,
       `ContainerForumId`: Long,