Workspace emptyvalues must have default values

When loading a 0.18.1 JASP file, the workspace default values are empty.
They should be set to the preferences empty values as default (since there is no workspace empty values in 0.18.1).
In DataSet::setEmptyValuesJson, there was already an handling to overcome this issue, but as DataSet has no access to the PreferencesModel object, it was depending of setting the _defaultEmptyvalues property just after the DataSet was created. But as DataSet might be deleted and created several times, the _defaultEmptyvalues is not always set. So just make it a static property, so that it just has to be set once.

CommonData/dataset.cpp

@@ -3,6 +3,8 @@
 #include <regex>
 #include "databaseinterface.h"
 
+stringset DataSet::_defaultEmptyvalues;
+
 DataSet::DataSet(int index)
 	: DataSetBaseNode(dataSetBaseNodeType::dataSet, nullptr)
 {

CommonData/dataset.h

@@ -101,7 +101,7 @@ class DataSet : public DataSetBaseNode
 	EmptyValues					_emptyValues;
 	bool						_writeBatchedToDB		= false,
 								_dataFileSynch			= false;
-	stringset					_defaultEmptyvalues;	// Default empty values if workspace do not have its own empty values (used for backward compatibility)
+	static stringset			_defaultEmptyvalues;	// Default empty values if workspace do not have its own empty values (used for backward compatibility)
 	std::string					_description;
 };