It is now possible to match empty fields

The VSL query could in theory do that, but I don't think it was
actionable without double quotes to delimit empty fields or fields
containing spaces.

bin/varnishtest/tests/r02872.vtc

@@ -39,3 +39,8 @@ shell -expect "Went healthy" {
         varnishlog -d -n ${v1_name} -g raw \
 		-q 'Backend_health[10] eq "HTTP/1.1 200 OK"'
 }
+
+# s1 starts sick before the first probe request is made
+shell -expect "Went sick" {
+        varnishlog -d -n ${v1_name} -g raw -q 'Backend_health[10] eq ""'
+}