Fix group v1 migration with empty group database file

scm-webapp/src/main/java/sonia/scm/update/group/XmlGroupV1UpdateStep.java

@@ -63,8 +63,10 @@ public class XmlGroupV1UpdateStep implements UpdateStep {
       return;
     }
     XmlGroupV1UpdateStep.V1GroupDatabase v1Database = readV1Database(v1GroupsFile.get());
+    if (v1Database.groupList != null && v1Database.groupList.groups != null) {
       v1Database.groupList.groups.forEach(this::update);
     }
+  }
 
   @Override
   public Version getTargetVersion() {

scm-webapp/src/test/java/sonia/scm/update/UpdateStepTestUtil.java

@@ -47,6 +47,12 @@ private final SCMContextProvider contextProvider;
     copyTestDatabaseFile(configDir, fileName);
   }
 
+  public void copyConfigFile(String fileName, String targetFileName) throws IOException {
+    Path configDir = tempDir.resolve("config");
+    Files.createDirectories(configDir);
+    copyTestDatabaseFile(configDir, fileName, targetFileName);
+  }
+
   public ConfigurationEntryStore<AssignedPermission> getStoreForConfigFile(String name) {
     return storeFactory
       .withType(AssignedPermission.class)
@@ -59,7 +65,12 @@ private final SCMContextProvider contextProvider;
   }
 
   private void copyTestDatabaseFile(Path configDir, String fileName) throws IOException {
+    Path targetFileName = Paths.get(fileName).getFileName();
+    copyTestDatabaseFile(configDir, fileName, targetFileName.toString());
+  }
+
+  private void copyTestDatabaseFile(Path configDir, String fileName, String targetFileName) throws IOException {
     URL url = Resources.getResource(fileName);
-    Files.copy(url.openStream(), configDir.resolve(Paths.get(fileName).getFileName()));
+    Files.copy(url.openStream(), configDir.resolve(targetFileName));
   }
 }

scm-webapp/src/test/java/sonia/scm/update/group/XmlGroupV1UpdateStepTest.java

@@ -99,6 +99,36 @@ class XmlGroupV1UpdateStepTest {
     }
   }
 
+  @Nested
+  class WithExistingDatabaseWithEmptyList {
+
+    @BeforeEach
+    void createGroupV1XML() throws IOException {
+      testUtil.copyConfigFile("sonia/scm/update/group/groups_empty_groups.xml", "groups.xml");
+    }
+
+    @Test
+    void shouldCreateNewGroupFromGroupsV1Xml() throws JAXBException {
+      updateStep.doUpdate();
+      verify(groupDAO, times(0)).add(any());
+    }
+  }
+
+  @Nested
+  class WithExistingDatabaseWithoutList {
+
+    @BeforeEach
+    void createGroupV1XML() throws IOException {
+      testUtil.copyConfigFile("sonia/scm/update/group/groups_no_groups.xml", "groups.xml");
+    }
+
+    @Test
+    void shouldCreateNewGroupFromGroupsV1Xml() throws JAXBException {
+      updateStep.doUpdate();
+      verify(groupDAO, times(0)).add(any());
+    }
+  }
+
   @Test
   void shouldNotFailForMissingConfigDir() throws JAXBException {
     updateStep.doUpdate();

scm-webapp/src/test/resources/sonia/scm/update/group/groups_empty_groups.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<group-db>
+    <creationTime>1558006904769</creationTime>
+    <groups/>
+    <lastModified>1558007174172</lastModified>
+</group-db>

scm-webapp/src/test/resources/sonia/scm/update/group/groups_no_groups.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<group-db>
+    <creationTime>1558006904769</creationTime>
+    <lastModified>1558007174172</lastModified>
+</group-db>