tests: Fill fuzzing coverage gaps for functions in primitives/block.h

src/test/fuzz/block_header.cpp

@@ -38,4 +38,12 @@ void test_one_input(const std::vector<uint8_t>& buffer)
         block.SetNull();
         assert(block.GetBlockHeader().GetHash() == mut_block_header.GetHash());
     }
+    {
+        std::optional<CBlockLocator> block_locator = ConsumeDeserializable<CBlockLocator>(fuzzed_data_provider);
+        if (block_locator) {
+            (void)block_locator->IsNull();
+            block_locator->SetNull();
+            assert(block_locator->IsNull());
+        }
+    }
 }