Enforce call to context->param as scalar

This is a follow up to the problem fixed by #195.
When a parameter given in the argument list of a validator does
not exist this is evaluated as list context and the empty list
does not change the runtime_args with the consequence that the
position of the arguments changes in the list, breaking the
functionality of the validator.

Author: oliwel

lib/Workflow/Action.pm

@@ -80,7 +80,7 @@ sub validate {
         my @runtime_args = ($wf);
         foreach my $arg ( @{$args} ) {
             if ( $arg =~ /^\$(.*)$/ ) {
-                push @runtime_args, $context->param($1);
+                push @runtime_args, scalar $context->param($1);
             } else {
                 push @runtime_args, $arg;
             }