Fix error when writing empty String to DataBuffer

Prior to this commit, `DataBuffer.write` would throw an
`IllegalStateException` when called with an empty `String`, since the
`CharsetEncoder` would be flushed on an incorrent state.

This commit skips entirely the encoding phase for empty `String`.

Fixes #22262

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -179,6 +179,16 @@ public class DataBufferTests extends AbstractDataBufferAllocatingTestCase {
 		}
 	}
 
+	@Test
+	public void writeEmptyString() {
+		DataBuffer buffer = createDataBuffer(1);
+		buffer.write("", StandardCharsets.UTF_8);
+
+		assertEquals(0, buffer.readableByteCount());
+
+		release(buffer);
+	}
+
 	@Test
 	public void writeUtf8String() {
 		DataBuffer buffer = createDataBuffer(6);