Revision 1760101

[MWAR-396] Check the existence of the web.xml based on the existence of particular classes

In 3.0, the default value of 'failOnMissingWebXml' was changed to be 'false'. With this fix, the default value is set to 'null' so that:
 - when the parameter is explicitly specified, it is used;
 - when it is not specified, it defaults to whether or not we can detect a Servlet 3.0 or newer environment for the web application. Such an environment does not need a web.xml since it can be written in Java.