Set default isAvailable and isEnabled functions
Set default isAvailable and isEnabled functions

This way when we make an ExpressionSlot using either of those functions,
we won't get an error in ExpressionSlot because `code` was set to null.

foam-framework/foam2,jacksonic/vjlofvhjfgm,foam-framework/foam2,foam-framework/foam2,jacksonic/vjlofvhjfgm,jacksonic/vjlofvhjfgm,foam-framework/foam2,foam-framework/foam2