Warn on missing serverURL
Warn on missing serverURL

We see some issues that are based on not properly configured
`config.serverURL`.

This patch adds a warning when `config.serverURL` is an empty value.
This should provide users direct feedback about how to improve their
configs.

Signed-off-by: Sheogorath <1c2647ce2ee8a4ec8e5c8b1b245ca38f94844bab@shivering-isles.com>

Yukaii/hackmd,jackycute/HackMD,hackmdio/hackmd,hackmdio/hackmd,jackycute/HackMD,hackmdio/hackmd,Yukaii/hackmd,Yukaii/hackmd,jackycute/HackMD