Update NULL Topic

Author: LeonnardoVerol

docs/programming-languages/null/_go.mdx

@@ -1,3 +1,14 @@
 ```Go
-// TODO
+// Go uses nil for pointers, slices, maps, channels, functions, and interfaces.
+
+var pointer *int = nil
+var values []int = nil
+var users map[string]int = nil
+
+if pointer == nil {
+	// ...
+}
+
+// Basic value types do not use nil.
+var value int = 0
 ```

docs/programming-languages/null/_java.mdx

@@ -1,3 +1,15 @@
 ```Java
-// TODO
+String variableName = null;
+
+if (variableName == null) {
+	// ...
+}
+
+if (variableName != null) {
+	System.out.println(variableName.length());
+}
+
+// Optional can be used when absence is part of the API contract.
+Optional<String> optionalValue = Optional.empty();
+Optional<String> optionalValue = Optional.of("value");
 ```

docs/programming-languages/null/_rust.mdx

@@ -1,3 +1,16 @@
 ```Rust
-// TODO
+// Rust does not have null.
+// Use Option<T> when a value can be absent.
+
+let variable_name: Option<String> = None;
+let variable_name: Option<String> = Some(String::from("value"));
+
+if let Some(value) = variable_name {
+	println!("{}", value);
+}
+
+match variable_name {
+	Some(value) => println!("{}", value),
+	None => println!("No value"),
+}
 ```

docs/programming-languages/null/_typescript.mdx

@@ -9,7 +9,13 @@
 // Declaring nullable
 // ------------------------------------
 
-// TODO
+let variableName: string | null = null;
+let variableName: string | undefined = undefined;
+let variableName: string | null | undefined = null;
+
+function myFunction(value?: string): void {
+	// value is string | undefined
+}
 ```
 
 ```Typescript