Always return a value from Workflow::Base->param()

In 1.58 and 1.59, Workflow was changed to return an empty list instead of
`undef` when the param entry does not exist. This, however, causes two
regressions in LedgerSMB where `param` is called in list context, but
expected to return `undef` (as it used to before 1.58).

lib/Workflow/Base.pm

@@ -42,10 +42,7 @@ sub param {
     }
 
     unless ( defined $value ) {
-        if ( exists $self->{PARAMS}{$name} ) {
-            return $self->{PARAMS}{$name};
-        }
-        return;
+        return $self->{PARAMS}{$name};
     }
     return $self->{PARAMS}{$name} = $value;
 }