Travis: fix builds timing out

Since merge of GHSA-mcqj-7p29-9528 (7055731),
the builds are timing out during travis_before_script.sh "MantisBT
Installation" step, because curl is not returning any output.

This is due to to introduction of the new path parameter.

Setting it to the actual URL `http://$HOSTNAME:$PORT/` does not work, so
we use a blank path.

Fixes #33791

(cherry picked from commit cd2cf5b)

build/travis_before_script.sh

@@ -129,6 +129,7 @@ declare -A query=(
 	[admin_username]=$DB_USER
 	[admin_password]=$DB_PASSWORD
 	[timezone]=UTC
+	[path]=''
 )
 
 # Build http query string