fix(muscript): null-check in ExprUtils

muscript-ast/src/main/java/io/gitlab/jfronny/muscript/ast/context/ExprUtils.java

@@ -61,13 +61,14 @@ public class ExprUtils {
     }
 
     public static Expr unpack(Expr expression) {
-        return switch (expression) {
+        return unpack(switch (expression) {
+            case null -> throw new NullPointerException();
             case BoolUnpack(var inner) -> inner;
             case NumberUnpack(var inner) -> inner;
             case StringUnpack(var inner) -> inner;
             case DynamicCoerce(var inner) -> inner;
             case StringCoerce(var inner) -> inner;
             default -> expression;
-        };
+        });
     }
 }