Due to the order of operations for boolean logic in java, this line of code gets evaluated as
(event.getDrops().isEmpty() && block == Blocks.GLASS) || (block == Blocks.STAINED_GLASS && !event.isSilkTouching())
Instead of
event.getDrops().isEmpty() && (block == Blocks.GLASS || block == Blocks.STAINED_GLASS) && !event.isSilkTouching()
Look closely at the parentheses. Currently, it will always drops glass shards for stained glass as long as the item in your hand isn't enchanted with silk touch. It doesn't care if event.getDrops() is empty or not.