http2: fix endless loop when write an empty string

If we do `req.write("")` (an empty string) on Http2Stream, the gather
logic will be messed up:

```
while ((src_length = nghttp2_session_mem_send(session_, &src)) > 0) {
  printf("write\n");
  DEBUG_HTTP2SESSION2(this, "nghttp2 has %d bytes to send", src_length);
  CopyDataIntoOutgoing(src, src_length);
}
```

The logic above is in function `Http2Session::SendPendingData` under
src/node_http2.cc. This `while` will be endless when an empty string is
inside because `src_length` will always be 9.

And then the main event loop thread will be blocked and the memory used
will be larger and larger.

This pull request is to ignore empty string or buffer in `_write()` and
`_writev()` of Http2Stream.

Fixes: nodejs#18169
Refs: https://github.com/nodejs/node/blob/v9.5.0/src/node_http2.cc#L1481-L1484
Refs: https://github.com/nodejs/node/blob/v9.5.0/lib/_http_outgoing.js#L659-L661

lib/internal/http2/core.js

@@ -1622,6 +1622,11 @@ class Http2Stream extends Duplex {
     if (!this.headersSent)
       this[kProceed]();
 
+    if (!data.length) {
+      cb();
+      return;
+    }
+
     const handle = this[kHandle];
     const req = new WriteWrap();
     req.stream = this[kID];
@@ -1659,6 +1664,11 @@ class Http2Stream extends Duplex {
     if (!this.headersSent)
       this[kProceed]();
 
+    if (!data.length) {
+      cb();
+      return;
+    }
+
     const handle = this[kHandle];
     const req = new WriteWrap();
     req.stream = this[kID];