rename Material.isAir to Material.isEmpty

represents what its more for, and aligns with future Minecraft goals

Spigot-API-Patches/0157-Material-API-additions.patch

@@ -1,11 +1,11 @@
-From a73476b729503e004260a1b5c618ccdaec9678e7 Mon Sep 17 00:00:00 2001
+From 96b6ec1a816d21b6c3eaabce56ccfa7c9d44afdb Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Sat, 6 Oct 2018 21:14:29 -0400
 Subject: [PATCH] Material API additions
 
 
 diff --git a/src/main/java/org/bukkit/Material.java b/src/main/java/org/bukkit/Material.java
-index 83f8eca15..bf7313fdb 100644
+index 83f8eca15..98775ea54 100644
 --- a/src/main/java/org/bukkit/Material.java
 +++ b/src/main/java/org/bukkit/Material.java
 @@ -72,6 +72,7 @@ import org.bukkit.material.MaterialData;
@@ -25,7 +25,7 @@ index 83f8eca15..bf7313fdb 100644
 +    /**
 +     * @return If the type is either AIR, CAVE_AIR or VOID_AIR
 +     */
-+    public boolean isAir() {
++    public boolean isEmpty() {
 +        switch (this) {
 +            case AIR:
 +            case CAVE_AIR: