List operations sometimes fails

[pilhuhn]

@jmazzitelli and I saw this fly by:

Exception in thread "JavaFX Application Thread" org.jruby.exceptions.RaiseException: (NoMethodError) undefined method `[]' for nil:NilClass
    at RUBY.initialize(/Users/hrupp/.rvm/gems/jruby-9.1.2.0/gems/hawkular-client-2.4.0/lib/hawkular/inventory/entities.rb:122)
    at RUBY.block in list_operation_definitions(/Users/hrupp/.rvm/gems/jruby-9.1.2.0/gems/hawkular-client-2.4.0/lib/hawkular/inventory/inventory_api.rb:418)
    at org.jruby.RubyArray.each(org/jruby/RubyArray.java:1593)
    at RUBY.list_operation_definitions(/Users/hrupp/.rvm/gems/jruby-9.1.2.0/gems/hawkular-client-2.4.0/lib/hawkular/inventory/inventory_api.rb:417)
    at RUBY.list_operation_definitions_for_resource(/Users/hrupp/.rvm/gems/jruby-9.1.2.0/gems/hawkular-client-2.4.0/lib/hawkular/inventory/inventory_api.rb:429)
    at RUBY.add_operations(/Users/hrupp/src/hawkfx/lib/on_click_cell_factory.rb:130)
    at RUBY.block in initialize(/Users/hrupp/src/hawkfx/lib/on_click_cell_factory.rb:51)

Looking at the code shows param_list = op_hash['properties']['params'],
so it looks like the received entity sometimes has no properties (or worse: H-inventory sends us some empty hashes).