Change some null validations

Replaced some code structures of this type:
if(x == null) {
    throws IllegalArgumentException(x, message);
}
with:
Validate.notNull(x, message);
in:
https://github.com/Bukkit/CraftBukkit/blob/master/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java

https://github.com/Bukkit/CraftBukkit/blob/master/src/main/java/org/bukkit/craftbukkit/entity/CraftHumanEntity.java

https://github.com/Bukkit/CraftBukkit/blob/master/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java