Handler error if a header definition is empty
If you create an header with an empty value, this generates an error:
[Wed Aug 28 14:45:02.729119 2019] [perl:error] [pid 52549]
[Wed Aug 28 14:45:02.729210 2019] [perl:error] [pid 52549] Lemonldap::NG::Handler::ApacheMP2::Main Unable to forge test1.openid.club headers:
Even this should not happen, we should not fail on this. The header could be ignored, or sent with an empty value.