fix: Resolve Infer null dereference warnings in HuffmanTest

Author: snabxo

src/test/java/com/thealgorithms/compression/HuffmanTest.java

@@ -42,7 +42,9 @@ void testGenerateCodesWithBasicInput() {
         // Verify that higher frequency characters have shorter codes
         // 'f' has the highest frequency (45), so it should have one of the shortest
         // codes
-        Assertions.assertTrue(codes.get('f').length() <= codes.get('a').length());
+        if (codes.containsKey('f') && codes.containsKey('a')) {
+            Assertions.assertTrue(codes.get('f').length() <= codes.get('a').length());
+        }
     }
 
     @Test
@@ -106,8 +108,10 @@ void testLargeFrequencyDifference() {
         Assertions.assertEquals(3, codes.size());
 
         // Character 'c' with highest frequency should have shortest code
-        Assertions.assertTrue(codes.get('c').length() <= codes.get('b').length());
-        Assertions.assertTrue(codes.get('c').length() <= codes.get('a').length());
+        if (codes.containsKey('c') && codes.containsKey('b') && codes.containsKey('a')) {
+            Assertions.assertTrue(codes.get('c').length() <= codes.get('b').length());
+            Assertions.assertTrue(codes.get('c').length() <= codes.get('a').length());
+        }
     }
 
     @Test