fix: #35 - Adds nil check in case backtrace is empty

Author: bougyman

lib/leopard/errors.rb

@@ -9,7 +9,7 @@ def initialize(...)
       end
 
       def backtrace
-        super[0..3] + ['... (truncated by Leopard)']
+        (super || [])[0..3] + ['... (truncated by Leopard)']
       end
     end