on resume check if the runout sensor actually exists before checking …

[pellcorp]

…its enabled or filament detected

This is a simple change to check for the filament sensor being defined, I encountered an issue where the _CLIENT_VARIABLES had the runout_sensor config, but it had been commented out in the printer.cfg, this simple change allows for this easy misconfiguration without ruining a print due to a failed resume which can never be fixed without a restart.

The error we get when we try to resume if the runout_sensor is defined but does not match a actual filament sensor in the rest of printer.cfg is

$ RESUME
05:22:44
!! Error evaluating 'gcode_macro RESUME:gcode': jinja2.exceptions.UndefinedError: 'klippy.extras.gcode_macro.GetStatusWrapperJinja object' has no attribute 'filament_switch_sensor filament_sensor'