Fix NPE issue when batch execution is called without any queries (#207)

Motivation:
(Please describe the problem you are trying to solve, or the new feature
you are trying to add.)

we're using this R2DBC client with jOOQ.
NPE occurs when we give empty list to `DSLContext#batch`

```kotlin
val queries = emptyList()
dslContext.batch(queries).awaitLast()
```

this is because `StringBuilder` in `MySqlBatchingBatch` is not
initialized until `add` method is called.
so, `execute` method will refer to not-initialized `builder` when `add`
method is not called.

Modification:
(Please describe the changes you have made to the codebase, including
any new files, modified files, or deleted files.)

~~modify `execute` to return `Flux#empty` when ` builder` is not
initialized (i.e., `add` method is not called).~~
send empty string to client without NPE

Result:
(Please describe the expected outcome of your changes, and any potential
side effects or drawbacks.
If possible, please also include any relevant testing results.)

it now does not throw NPE ~~but returns empty flux instead.~~

src/main/java/io/asyncer/r2dbc/mysql/MySqlBatchingBatch.java

@@ -20,6 +20,8 @@
 import io.asyncer.r2dbc.mysql.codec.Codecs;
 import reactor.core.publisher.Flux;
 
+import java.util.StringJoiner;
+
 import static io.asyncer.r2dbc.mysql.internal.util.AssertUtils.requireNonNull;
 
 /**
@@ -34,7 +36,7 @@ final class MySqlBatchingBatch extends MySqlBatch {
 
     private final ConnectionContext context;
 
-    private StringBuilder builder;
+    private final StringJoiner queries = new StringJoiner(";");
 
     MySqlBatchingBatch(Client client, Codecs codecs, ConnectionContext context) {
         this.client = requireNonNull(client, "client must not be null");
@@ -50,9 +52,9 @@ public MySqlBatch add(String sql) {
 
         if (index >= 0 && sql.charAt(index) == ';') {
             // Skip last ';' and whitespaces that following last ';'.
-            requireBuilder().append(sql, 0, index);
+            queries.add(sql.substring(0, index));
         } else {
-            requireBuilder().append(sql);
+            queries.add(sql);
         }
 
         return this;
@@ -75,15 +77,7 @@ public String toString() {
      * @return current batching SQL statement
      */
     String getSql() {
-        return builder.toString();
-    }
-
-    private StringBuilder requireBuilder() {
-        if (builder == null) {
-            return (builder = new StringBuilder());
-        }
-
-        return builder.append(';');
+        return queries.toString();
     }
 
     private static int lastNonWhitespace(String sql) {

src/test/java/io/asyncer/r2dbc/mysql/MySqlBatchingBatchTest.java

@@ -59,4 +59,11 @@ void add() {
     void badAdd() {
         assertThrows(IllegalArgumentException.class, () -> batch.add(null));
     }
+
+    @Test
+    void addNothing() {
+        final MySqlBatchingBatch batch = new MySqlBatchingBatch(mock(Client.class), mock(Codecs.class),
+            ConnectionContextTest.mock());
+        assertEquals(batch.getSql(), "");
+    }
 }