Added a test case for null results

Author: jr20xx

jcalc/src/test/java/cu/lt/joe/jcalc/JCalcTest.java

@@ -1,13 +1,20 @@
 package cu.lt.joe.jcalc;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvFileSource;
 
 public class JCalcTest
 {
+    @Test
+    void testNullResult()
+    {
+        assertNull(JCalc.solveMathExpression(null));
+    }
+
     @Test
     void testIllegalArgumentException()
     {