Silence an uninitialized variable warning from gen-usage()

src/core/Main.pm

@@ -125,7 +125,7 @@ my sub MAIN_HELPER($retval = 0) is hidden_from_backtrace {
                     }
                 }
                 else {
-                    my $constraints  = ~$param.constraint_list;
+                    my $constraints  = $param.constraint_list.map(*.gist).join(' ');
                     my $simple-const = $constraints && $constraints !~~ /^_block/;
                     $argument = $param.name   ?? '<' ~ $param.name.substr(1) ~ '>' !!
                                 $simple-const ??       $constraints                !!