fix: StringUtil join null error (#165)

Author: YongwuHe

arex-agent-bootstrap/src/main/java/io/arex/agent/bootstrap/util/StringUtil.java

@@ -43,7 +43,7 @@ public static String join(final Iterable<?> iterable, final String separator) {
         Iterator<?> iterator = iterable.iterator();
 
         if (!iterator.hasNext()) {
-            return null;
+            return EMPTY;
         }
 
         final Object first = iterator.next();

arex-agent-bootstrap/src/test/java/io/arex/agent/bootstrap/util/StringUtilTest.java

@@ -64,7 +64,7 @@ void join() {
         assertNull(actualResult);
 
         actualResult = StringUtil.join(Collections.emptyList(), ",");
-        assertNull(actualResult);
+        assertEquals(StringUtil.EMPTY, actualResult);
 
         actualResult = StringUtil.join(Collections.singleton("a"), ",");
         assertEquals("a", actualResult);

arex-instrumentation-api/src/test/java/io/arex/inst/runtime/util/TypeUtilTest.java

@@ -220,4 +220,13 @@ public T getValue() {
 
     }
 
+    @Test
+    public void testNullList() {
+        final List<Object> list = new ArrayList<>();
+        list.add(null);
+        String expectedName = "java.util.ArrayList-";
+        final String actualName = TypeUtil.getName(list);
+        assertEquals(expectedName, actualName);
+    }
+
 }

arex-instrumentation-foundation/src/test/java/io/arex/foundation/serializer/JacksonSerializerTest.java

@@ -2,7 +2,10 @@
 
 import static org.junit.jupiter.api.Assertions.*;
 
+import io.arex.inst.runtime.util.TypeUtil;
 import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
 import org.junit.jupiter.api.Test;
 
 class JacksonSerializerTest {
@@ -22,4 +25,16 @@ void testLocalTime() {
         assertEquals(now, actualResult);
     }
 
+    @Test
+    void testNullList() {
+        final List<Object> list = new ArrayList<>();
+        list.add(null);
+        String json = JacksonSerializer.INSTANCE.serialize(list);
+        final String name = TypeUtil.getName(list);
+        final List<Object> result = JacksonSerializer.INSTANCE.deserialize(json, TypeUtil.forName(name));
+        assert result != null;
+        assertEquals(list.size(), result.size());
+        assertNull(result.get(0));
+    }
+
 }