Java: Add test for NullLiteral

Author: Marcono1234

java/ql/lib/semmle/code/java/Expr.qll

@@ -734,6 +734,7 @@ class StringLiteral extends Literal, @stringliteral {
 
 /** The null literal, written `null`. */
 class NullLiteral extends Literal, @nullliteral {
+  // Override these predicates because the inherited ones have no result
   override string getLiteral() { result = "null" }
 
   override string getValue() { result = "null" }

java/ql/test/library-tests/literals/nullLiterals/NullLiterals.java

@@ -0,0 +1,22 @@
+package nullLiterals;
+
+public class NullLiterals {
+	Object[] nulls = {
+		null,
+		// Using Unicode escapes (which are handled during pre-processing)
+		\u006E\u0075\u006C\u006C, // null
+	};
+
+	// The operation expressions (e.g. cast) are not a literal
+	Object[] operations = {
+		(Object) null,
+	};
+
+	Object[] nonNullLiterals = {
+		"null",
+		0,
+		Boolean.FALSE,
+	};
+
+	Object nonLiteral;
+}

java/ql/test/library-tests/literals/nullLiterals/nullLiterals.expected

@@ -0,0 +1,3 @@
+| NullLiterals.java:5:3:5:6 | null | null |
+| NullLiterals.java:7:8:7:26 | null | null |
+| NullLiterals.java:12:12:12:15 | null | null |

java/ql/test/library-tests/literals/nullLiterals/nullLiterals.ql

@@ -0,0 +1,4 @@
+import semmle.code.java.Expr
+
+from NullLiteral lit
+select lit, lit.getValue()