SPIGOT-5341: Add Material.isAir

By: md_5 <git@md-5.net>

paper-server/src/test/java/org/bukkit/PerMaterialTest.java

@@ -195,6 +195,15 @@ public class PerMaterialTest extends AbstractTestingBase {
         }
     }
 
+    @Test
+    public void testAir() {
+        if (material.isBlock()) {
+            assertThat(material.isAir(), is(equalTo(CraftMagicNumbers.getBlock(material).getBlockData().isAir())));
+        } else {
+            assertThat(material.isAir(), is(equalTo(false)));
+        }
+    }
+
     @Test
     public void testItem() {
         if (material == Material.AIR) {