Fix Java IterableByteBufferInputStream to return -1 on empty

This gives the InputStream slightly less work to do when it knows there
are no buffers that can be checked for more data.

Also loosened a check in CodedInputStream to permit read() to return
zero, as this is documented as a valid value - though in this case -1 is
the correct value as there is nothing more to read.

Fixes #23957

Author: niloc132

java/core/src/main/java/com/google/protobuf/CodedInputStream.java

@@ -2889,7 +2889,7 @@ private boolean tryRefillBuffer(int n) throws IOException {
                   buffer.length - bufferSize,
                   //  do not exceed the total bytes limit
                   sizeLimit - totalBytesRetired - bufferSize));
-      if (bytesRead == 0 || bytesRead < -1 || bytesRead > buffer.length) {
+      if (bytesRead < -1 || bytesRead > buffer.length) {
         throw new IllegalStateException(
             input.getClass()
                 + "#read(byte[]) returned invalid result: "

java/core/src/main/java/com/google/protobuf/IterableByteBufferInputStream.java

@@ -62,7 +62,7 @@ final class IterableByteBufferInputStream extends InputStream {
 
     if (!getNextByteBuffer()) {
       currentByteBuffer = EMPTY_BYTE_BUFFER;
-      currentIndex = 0;
+      currentIndex = dataSize;
       currentByteBufferPos = 0;
       currentAddress = 0;
     }

java/core/src/test/java/com/google/protobuf/IterableByteBufferInputStreamTest.java

@@ -37,4 +37,19 @@ public void testEmptyBuffers() throws Exception {
       assertThat(in.read()).isEqualTo(-1);
     }
   }
+
+  @Test
+  public void testAllEmpty() throws Exception {
+    verifyEmpty();
+    verifyEmpty(ByteBuffer.wrap(new byte[0]));
+    verifyEmpty(ByteBuffer.wrap(new byte[0]),
+            ByteBuffer.wrap(new byte[0]),
+            ByteBuffer.wrap(new byte[0]));
+  }
+
+  private static void verifyEmpty(ByteBuffer... buffers) throws Exception {
+    try (InputStream in = new IterableByteBufferInputStream(Arrays.asList(buffers))) {
+      assertThat(in.read()).isEqualTo(-1);
+    }
+  }
 }