Fix some warnings.

src/main/java/edu/hm/hafner/grading/CoverageScore.java

@@ -5,7 +5,6 @@
 import java.util.List;
 import java.util.Objects;
 
-import org.apache.commons.lang3.ObjectUtils;
 import org.apache.commons.lang3.StringUtils;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
@@ -87,7 +86,7 @@ public String getMetricTagName() {
 
     @JsonIgnore
     public Node getReport() {
-        return ObjectUtils.defaultIfNull(report, new ModuleNode("empty"));
+        return report;
     }
 
     @Override

src/main/java/edu/hm/hafner/grading/ScoreMarkdown.java

@@ -30,6 +30,7 @@ abstract class ScoreMarkdown<S extends Score<S, C>, C extends Configuration> {
     static final int MESSAGE_INITIAL_CAPACITY = 1024;
     private static final int MAX_SIZE = 10_000; // limit the size of the output to this number of characters
     private static final String TRUNCATION_TEXT = "\n\nToo many test failures. Grading output truncated.";
+    private static final int HUNDRED_PERCENT = 100;
 
     private final String type;
     private final String icon;
@@ -50,7 +51,7 @@ abstract class ScoreMarkdown<S extends Score<S, C>, C extends Configuration> {
      * @return Markdown text
      */
     public static String getPercentageImage(final String title, final int percentage) {
-        if (percentage < 0 || percentage > 100) {
+        if (percentage < 0 || percentage > HUNDRED_PERCENT) {
             throw new IllegalArgumentException("Percentage must be between 0 and 100: " + percentage);
         }
         return String.format("""
@@ -150,8 +151,8 @@ static String createScoreTitleSuffix(final int maxScore, final int value, final
         if (maxScore == 0) {
             return StringUtils.EMPTY;
         }
-        if (maxScore == 100) {
-            return String.format(" - %d of %d", value, maxScore);
+        if (maxScore == HUNDRED_PERCENT) {
+            return String.format(" - %d of %d", value, maxScore); // no need to show percentage for a score of 100
         }
         return String.format(" - %d of %d (%d%%)", value, maxScore, percentage);
     }

src/main/java/edu/hm/hafner/grading/TestScore.java

@@ -7,7 +7,6 @@
 import java.util.function.Function;
 import java.util.stream.Collectors;
 
-import org.apache.commons.lang3.ObjectUtils;
 import org.apache.commons.lang3.StringUtils;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
@@ -36,8 +35,7 @@ public final class TestScore extends Score<TestScore, TestConfiguration> {
     private final int passedSize;
     private final int failedSize;
     private final int skippedSize;
-    @CheckForNull
-    private final transient Node report;
+    private transient Node report; // do not persist the tree of nodes
 
     private TestScore(final String id, final String name, final TestConfiguration configuration,
             final List<TestScore> scores) {
@@ -61,6 +59,18 @@ private TestScore(final String id, final String name, final TestConfiguration co
         skippedSize = sum(report, TestResult.SKIPPED);
     }
 
+    /**
+     * Restore an empty report after de-serialization.
+     *
+     * @return this
+     */
+    @Serial @CanIgnoreReturnValue
+    private Object readResolve() {
+        report = new ModuleNode("empty");
+
+        return this;
+    }
+
     private int aggregate(final List<TestScore> scores, final Function<TestScore, Integer> property) {
         return scores.stream().reduce(0, (sum, score) -> sum + property.apply(score), Integer::sum);
     }
@@ -75,7 +85,7 @@ private int sum(final Node testReport, final TestResult testResult) {
 
     @JsonIgnore
     public Node getReport() {
-        return ObjectUtils.defaultIfNull(report, new ModuleNode("empty"));
+        return report;
     }
 
     @Override