merge filesetmanifest=mergewithoutmain even if no explicit manifest has been specified.  PR 54171

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1555355 13f79535-47bb-0310-9956-ffa450edef68
add a null-guard PR 53754

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@1555356 13f79535-47bb-0310-9956-ffa450edef68