Improve throw-null-literal.ql

Tested query with Variant Analysis and had multiple results.

Author: Marcono1234

codeql-custom-queries-java/not-tested-queries/throw-null-literal.ql

@@ -0,0 +1,18 @@
+/**
+ * Finds `throw` statements which throw the `null` literal, i.e. `throw null;`.
+ * While this does produce a `NullPointerException` (as probably desired)
+ * it is rather obscure and misuses the `null` check of the `throw` statement.
+ * For clarity it would be better to explicitly create a `NullPointerException`
+ * and possibly provide a meaningful message to its constructor.
+ * 
+ * See also Error Prone pattern [ThrowNull](https://errorprone.info/bugpattern/ThrowNull).
+ * 
+ * @kind problem
+ */
+
+import java
+
+from ThrowStmt throwStmt
+where
+    throwStmt.getExpr() instanceof NullLiteral
+select throwStmt, "Should use `throw new NullPointerException(...)` instead"