Fix undefined displayData on barcode generation

[cedric-anne]

In #36, no displayData was added in barcode generation parameters. So when generation a barcode, following errors are trigerred.

I do not know if is is due to something missing in the PR or if it is normal (functionnal point of view). @xacobofg ?

[2019-11-08 10:31:33] glpiphplog.ERROR: Toolbox::userErrorHandlerNormal() in /var/www/glpi/inc/toolbox.class.php line 663
  *** PHP Notice(8): Undefined index: displayData
  Backtrace :
  plugins/barcode/inc/barcode.class.php:294          
  plugins/barcode/inc/barcode.class.php:512          PluginBarcodeBarcode->printPDF()
  inc/massiveaction.class.php:1027                   PluginBarcodeBarcode::processMassiveActionsForOneItemtype()
  inc/massiveaction.class.php:1006                   MassiveAction->processForSeveralItemtypes()
  front/massiveaction.php:59                         MassiveAction->process()
  {"user":"2@4028a755e155"} 
[2019-11-08 10:31:33] glpiphplog.ERROR: Toolbox::userErrorHandlerNormal() in /var/www/glpi/inc/toolbox.class.php line 663
  *** PHP Notice(8): Trying to access array offset on value of type null
  Backtrace :
  plugins/barcode/inc/barcode.class.php:348          
  plugins/barcode/inc/barcode.class.php:512          PluginBarcodeBarcode->printPDF()
  inc/massiveaction.class.php:1027                   PluginBarcodeBarcode::processMassiveActionsForOneItemtype()
  inc/massiveaction.class.php:1006                   MassiveAction->processForSeveralItemtypes()
  front/massiveaction.php:59                         MassiveAction->process()
  {"user":"2@4028a755e155","mem_usage":"0.002\", 10.19Mio)"} 

[trasher]

lgtm on a code pov

[xacobofg]

Errors are caused by the barcode not generating any data.
When I created the code I only considered the generation of labels for the QR.
I did not realize that since both use the same function to print, when trying to generate a barcode would cause an error to not have defined in the array displayData when sending the parameters.
I'm fine with the solution you're proposing.