Tests for various entity classes and update entity class implementations

js/entity/NullEntity.js

@@ -0,0 +1,16 @@
+import P from "parsernostrum"
+import IEntity from "./IEntity.js"
+
+export default class NullEntity extends IEntity {
+
+    static grammar = P.reg(new RegExp(String.raw`\(${P.whitespaceInlineOpt.getParser().regexp.source}\)`))
+        .map(v => new this())
+
+    toString(
+        insideString = false,
+        indentation = "",
+        printKey = this.Self().printKey,
+    ) {
+        return "()"
+    }
+}