made template parameter 'msg' optional

I've run into this before, in Mojolicious one can access a template parameter in two ways:

* as a variable in the package that is created for the template, eg $msg
* as the result of calling the stash function, eg stash('msg')

The latter option is more lenient, returning undef and not throwing an error if the parameter does not exist.

I'll wire up a template for index in the next commit.