core: silently ignore a not existent file path when IncludeOptional

     is used.

In https://bz.apache.org/bugzilla/show_bug.cgi?id=57585 some use cases
were reported in which IncludeOptional seems to be too strict in its
sanity checks.

This change is a proposal to relax IncludeOptional checks to silently
fail when a file path is not existent rather than returning SyntaxError.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1814968 13f79535-47bb-0310-9956-ffa450edef68