Fix Incorrect Command Cooldown Storage Warning (#8304)

fix incorrect warning

Author: erenkarakal

src/main/java/ch/njol/skript/command/ScriptCommand.java

@@ -533,7 +533,7 @@ public Date getLastUsage(UUID uuid, Event event) {
 			String name = getStorageVariableName(event);
 			assert name != null;
 			Object variable = Variables.getVariable(name, null, false);
-			if (!(variable instanceof Date)) {
+			if (variable != null && !(variable instanceof Date)) {
 				Skript.warning("Variable {" + name + "} was not a date! You may be using this variable elsewhere. " +
 						"This warning is letting you know that this variable is now overridden for the command storage.");
 				return null;