Remove empty command descriptions

src/main/java/com/volmit/iris/core/command/pregen/CommandIrisPregenStop.java

@@ -38,6 +38,6 @@ public class CommandIrisPregenStop extends MortarCommand {
 
     @Override
     protected String getArgsUsage() {
-        return null;
+        return "";
     }
 }

src/main/java/com/volmit/iris/core/command/pregen/CommandIrisPregenToggle.java

@@ -32,6 +32,6 @@ public class CommandIrisPregenToggle extends MortarCommand {
 
     @Override
     protected String getArgsUsage() {
-        return null;
+        return "";
     }
 }