Don't consume bonemeal if there is no crop in front. Fixes BUKKIT-3790

#### The Issue

A dispenser stocked with bonemeal will fertilize a crop in front of it. However, if a crop is not present the dispenser will still consume bonemeal. 

#### Justification for PR

The behaviour of the vanilla server is different, as it will not consume bonemeal if there is no crop. This behaviour also seems to make more sense.

#### PR Breakdown

The bone meal dispense behaviour was decrementing the itemstack, much like the other dispense behaviours. The difference is that ```ItemDye.a(itemstack, world, i, j, k)``` already handles the decrementing of the item stack. 

#### JIRA Ticket

BUKKIT-3790 - https://bukkit.atlassian.net/browse/BUKKIT-3790