Microoptimization of sendfile(2) usage under FreeBSD.

FreeBSD kernel checks headers/trailers pointer against NULL, not
corresponding count.  Passing NULL if there are no headers/trailers
helps to avoid unneeded work in kernel, as well as unexpected 0 bytes
GIO in traces.

src/os/unix/ngx_freebsd_sendfile_chain.c

diff --git a/src/os/unix/ngx_freebsd_sendfile_chain.c b/src/os/unix/ngx_freebsd_sendfile_chain.c
index 039243e..8c98f8e 100644
--- a/src/os/unix/ngx_freebsd_sendfile_chain.c
+++ b/src/os/unix/ngx_freebsd_sendfile_chain.c
@@ -246,9 +246,14 @@
                 }
             }
 
-            hdtr.headers = (struct iovec *) header.elts;
+            /*
+             * sendfile() does unneeded work if sf_hdtr's count is 0,
+             * but corresponding pointer is not NULL
+             */
+
+            hdtr.headers = header.nelts ? (struct iovec *) header.elts: NULL;
             hdtr.hdr_cnt = header.nelts;
-            hdtr.trailers = (struct iovec *) trailer.elts;
+            hdtr.trailers = trailer.nelts ? (struct iovec *) trailer.elts: NULL;
             hdtr.trl_cnt = trailer.nelts;
 
             /*