Use null instead of "N/A" as default change-number

Missing change.id is more clear to the consumer than interpreting a
string when change-number isn't available.

Solves: Jira GER-1698
Change-Id: Ic801fe799ba53d363306c443cb3e92993452ef65

src/main/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EiffelEventFactory.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EiffelEventFactory.java b/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EiffelEventFactory.java
index ef3547c..df4f63c 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EiffelEventFactory.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EiffelEventFactory.java
@@ -193,7 +193,7 @@
             .change(
                 EiffelChangeInfo.builder()
                     .details(commitUrl(repoName, commitSha1))
-                    .id(changeNbr != null ? String.valueOf(changeNbr) : "N/A")
+                    .id(changeNbr != null ? String.valueOf(changeNbr) : null)
                     .tracker(GERRIT))
             .meta(getMeta(epochMillisCreated));
     parentEventIds.stream()

src/test/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EventMappingIT.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EventMappingIT.java b/src/test/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EventMappingIT.java
index 575e1aa..ee2b553 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EventMappingIT.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/eventseiffel/mapping/EventMappingIT.java
@@ -102,7 +102,7 @@
     assertPersonInfo(event.data.author, AUTHOR_NAME, AUTHOR_EMAIL, null);
     assertGitIdentifier(
         event.data.gitIdentifier, project.get(), "master", accountlessAuthor.getName());
-    assertChangeInfo(event.data.change, "N/A", project.get(), accountlessAuthor.getName());
+    assertChangeInfo(event.data.change, null, project.get(), accountlessAuthor.getName());
     assertSccMeta(event.meta, commitTimeInEpochMillis(accountlessAuthor));
     assertSccLinks(event.links);
   }
@@ -118,7 +118,7 @@
     assertPersonInfo(event.data.author, admin.fullName(), admin.email(), admin.username());
     assertGitIdentifier(
         event.data.gitIdentifier, project.get(), "master", authorWithAccount.getName());
-    assertChangeInfo(event.data.change, "N/A", project.get(), authorWithAccount.getName());
+    assertChangeInfo(event.data.change, null, project.get(), authorWithAccount.getName());
     assertSccMeta(event.meta, commitTimeInEpochMillis(authorWithAccount));
     assertSccLinks(event.links);
   }