Minor ux improvements

Author: Matt Muller

gems/smithy/lib/smithy/views/client/endpoint_parameter.rb

@@ -146,6 +146,7 @@ def client_context_param_value
 
         def built_in_param_value
           return unless @data['builtIn']
+          raise ArgumentError, "Endpoint built-in not available for #{@data['builtIn']}" unless built_in_binding
 
           [built_in_binding[:render_build].call(@plan, nil), 'config']
         end

gems/smithy/lib/smithy/views/client/operation_examples.rb

@@ -66,6 +66,8 @@ def error_example(example)
         end
 
         def params(document, target, indent = '  ')
+          return '{}' if document.nil? || document.empty?
+
           shape = Model.shape(@model, target)
           structure(document, shape, indent).join("\n")
         end