This is a root cause for failing outerloop test: Public_Interop_Upgrade_Request3OrLower_Success for https://www.litespeedtech.com/

The server sends this Alt-Svc header:

Alt-Svc: =":443"; ma=2592000, h3=":443"; ma=2592000, h3-29=":443"; ma=2592000, h3-Q050=":443"; ma=2592000, h3-Q046=":443"; ma=2592000, h3-Q043=":443"; ma=2592000, quic=":443"; ma=2592000; v="43,46"

Note the first item is missing ALPN. Our header parsing then discards the whole header. RFC says that empty ALPN is not allowed as it specifies it as token

Firefox has no issues with this and is able to process the next request via h3.

Question is whether we should allow this? We could just ignore the unparsable value or allow empty ALPN.