http2: fix endless loop when writing empty string

PR-URL: https://github.com/nodejs/node/pull/18924
Fixes: https://github.com/nodejs/node/issues/18169
Refs: https://github.com/nodejs/node/pull/18673
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


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>