Fix warnings in DAOLogger (#2738)

We had a bunch of uninitialized values here,
and they were triggering warnings I did not see earlier
because I don't usually run CATALYST_DEBUG.

lib/Plack/Middleware/Debug/DAOLogger.pm

@@ -19,7 +19,7 @@ sub run {
     $i = 0;
 
     return sub {
-        my $sum = sum(map { $_->[1] } @call_stack);
+        my $sum = sum(map { $_->[1] } @call_stack) // 0;
 
         $panel->content(
             '<p>This panel shows time spent within the data access layer. Rows '.
@@ -39,7 +39,7 @@ sub run {
 
 sub render_stack {
     my ($indent, @stack) = @_;
-    return unless @stack;
+    return '' unless @stack;
 
     my @times = map { $_->[1] } @stack;
     my $mean = mean(@times);