Show the right implicit empty values in tooltip

If you set a nominal or ordinal variable in a scale VariablesList, a tooltip tells you how the values are transformed into scale.
It gives also a list of 'implicit' empty values: that is the not empty values that could not be converted into a double value. This list was not correct, it gave all the convertable values.

CommonData/column.cpp

@@ -2372,20 +2372,27 @@ stringvec Column::previewTransform(columnType transformType)
 	}
 
 	{
-		std::stringstream someEmptyValues;
+		std::stringstream someImplicitEmptyValues;
 
 		if(transformType == columnType::scale && labelsTempCount() > _labelsTempNumerics)
 		{
 			int count = 0;
 
 			for(Label * label : _labels)
-				if(!label->isEmptyValue() && count < showThisMany)
-					someEmptyValues << (count++ > 0 ? ", " : "") << '"' << label->originalValueAsString() << '"';
-				else if(!label->isEmptyValue() && count++ == showThisMany)
-					someEmptyValues << ", ...";
+			{
+				if(!label->isEmptyValue() && !ColumnUtils::isDoubleValue(label->label()))
+				{
+					if(count < showThisMany)
+						someImplicitEmptyValues << (count++ > 0 ? ", " : "") << '"' << label->originalValueAsString() << '"';
+					else if(count++ == showThisMany)
+						someImplicitEmptyValues << ", ...";
+					else
+						break; // Do not need to loop further over the labels.
+				}
+			}
 		}
 
-		out.push_back(someEmptyValues.str());
+		out.push_back(someImplicitEmptyValues.str());
 	}
 
 	return out;