Fix toString() method that could return null in NamedGroupsGuidelineCheckResult

Changed toString() to return a default message instead of null when no
specific named groups information is available. This prevents potential
NullPointerException when the toString() result is used.

Added comprehensive unit tests to verify toString() never returns null
for all possible states of the NamedGroupsGuidelineCheckResult object.

Author: TLS Scanner Developer

TLS-Server-Scanner/src/main/java/de/rub/nds/tlsscanner/serverscanner/guideline/results/NamedGroupsGuidelineCheckResult.java

@@ -69,7 +69,7 @@ public String toString() {
         if (groupCount != null) {
             return "Server only supports " + groupCount + " groups.";
         }
-        return null;
+        return "No specific named groups information available.";
     }
 
     public Set<NamedGroup> getNotRecommendedGroups() {

TLS-Server-Scanner/src/test/java/de/rub/nds/tlsscanner/serverscanner/guideline/results/NamedGroupsGuidelineCheckResultTest.java

@@ -0,0 +1,65 @@
+/*
+ * TLS-Scanner - A TLS configuration and analysis tool based on TLS-Attacker
+ *
+ * Copyright 2017-2023 Ruhr University Bochum, Paderborn University, Technology Innovation Institute, and Hackmanit GmbH
+ *
+ * Licensed under Apache License, Version 2.0
+ * http://www.apache.org/licenses/LICENSE-2.0.txt
+ */
+package de.rub.nds.tlsscanner.serverscanner.guideline.results;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import de.rub.nds.scanner.core.guideline.GuidelineAdherence;
+import de.rub.nds.tlsattacker.core.constants.NamedGroup;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
+import org.junit.jupiter.api.Test;
+
+class NamedGroupsGuidelineCheckResultTest {
+
+    @Test
+    void testToStringNeverReturnsNull() {
+        // Test with CHECK_FAILED adherence
+        NamedGroupsGuidelineCheckResult result1 =
+                new NamedGroupsGuidelineCheckResult("test1", GuidelineAdherence.CHECK_FAILED);
+        assertNotNull(result1.toString());
+        assertTrue(result1.toString().contains("Missing information"));
+
+        // Test with ADHERED adherence
+        NamedGroupsGuidelineCheckResult result2 =
+                new NamedGroupsGuidelineCheckResult("test2", GuidelineAdherence.ADHERED);
+        assertNotNull(result2.toString());
+        assertTrue(result2.toString().contains("Server passed"));
+
+        // Test with not recommended groups
+        Set<NamedGroup> notRecommended = new HashSet<>();
+        notRecommended.add(NamedGroup.SECP224R1);
+        NamedGroupsGuidelineCheckResult result3 =
+                new NamedGroupsGuidelineCheckResult(
+                        "test3", GuidelineAdherence.VIOLATED, notRecommended);
+        assertNotNull(result3.toString());
+        assertTrue(result3.toString().contains("not recommended"));
+
+        // Test with missing required groups
+        NamedGroupsGuidelineCheckResult result4 =
+                new NamedGroupsGuidelineCheckResult(
+                        "test4", GuidelineAdherence.VIOLATED, Arrays.asList(NamedGroup.SECP256R1));
+        assertNotNull(result4.toString());
+        assertTrue(result4.toString().contains("missing one of required"));
+
+        // Test with group count
+        NamedGroupsGuidelineCheckResult result5 =
+                new NamedGroupsGuidelineCheckResult("test5", GuidelineAdherence.VIOLATED, 3);
+        assertNotNull(result5.toString());
+        assertTrue(result5.toString().contains("only supports 3 groups"));
+
+        // Test default case - this is the case that was returning null
+        NamedGroupsGuidelineCheckResult result6 =
+                new NamedGroupsGuidelineCheckResult("test6", GuidelineAdherence.VIOLATED);
+        assertNotNull(result6.toString());
+        assertTrue(result6.toString().contains("No specific named groups information"));
+    }
+}