Added some null safety samples

material/samples/NullSafety.java

@@ -0,0 +1,18 @@
+// program that illustrates null safety in java
+public class NullSafety {
+    public static void main(String[] args) {
+        @NotNull
+        String name;
+        name = null; //
+        String name2;
+        name2 = null;
+        System.out.println(name2.length()); // compile error
+        System.out.println(name2 == null ? 0 : name2.length());
+    }
+
+    public static int getLength2(String name) {
+        if (name == null)
+            return 0;
+        return name.length();
+    }
+}

material/samples/NullSafety.kt

@@ -0,0 +1,13 @@
+fun main() {
+    var name: String
+    name = null // compile error
+    val name2: String?
+    name2 = null
+    println(name2.length) // compile error
+    print(name?.length ?: 0)
+}
+
+fun getLength2(name: String?): Int {
+    if (name == null) return 0
+    return name.length
+}