added description and resources for null in javascript (#6242)

src/data/roadmaps/javascript/content/null@CxyNyFMuTLS3owtRMgClD.md

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 # null
+
+The `null` value in JavaScript signifies the deliberate absence of any object value. It is considered as one of JavaScript's primitive values and a `falsy` value.
+
+*Deliberate absence* emphasises the intentional use of `null` to indicate that a variable does not point to any object. This explicit declaration conveys the purposeful nature of null, showing that the variable is meant to be empty or non-existent at execution time.
+
+In essence, `null` is a way to reset a variable, signalling that it should not reference any object.
+
+Visit the following resources to learn more:
+
+- [@article@What is null in JavaScript](https://www.altcademy.com/blog/what-is-null-in-javascript/)
+- [@article@null in JavaScript](https://masteringjs.io/tutorials/fundamentals/null)