Allow header_rewrite to remove query parameters

[djcarlin]

header_rewrite config:

set-destination QUERY ""

result:

[Nov 29 22:45:49.326] {0x2af11f66d700} DIAG: (header_rewrite) Would set destination QUERY to an empty value, skipping

The docs actually mention this - but it seems useful in this case:

You must supply a non-zero length value, otherwise this operator will be an effective no-op (though a warning will be emitted to the logs if debugging is enabled).

https://docs.trafficserver.apache.org/en/latest/admin-guide/plugins/header_rewrite.en.html#set-destination