Apache Tomcat: AJP response header mix-up

A regression in the fix for bug 66512 in Apache Tomcat 11.0.0-M5, 10.1.8, 9.0.74 and 8.5.88 meant that, if a response did not include any HTTP headers no AJP SEND_HEADERS messare woudl be sent for the response which in turn meant that at least one AJP proxy (mod_proxy_ajp) would use the response headers from the previous request leading to an information leak.