Added a check to return null if the provided Math expression is null

Author: jr20xx

jcalc/src/main/java/cu/lt/joe/jcalc/JCalc.java

@@ -42,6 +42,7 @@ public static String solveMathExpression(String mathExpression, ConfigurationBui
     {
         if (configurationBuilder == null)
             configurationBuilder = new ConfigurationBuilder();
-        return ShuntingYardAlgImpl.solveMathExpression(mathExpression, configurationBuilder.isBalanceParenthesesEnabled(), configurationBuilder.getPrecision(), configurationBuilder.isUseRadiansEnabled());
+        return mathExpression == null ? null :
+                ShuntingYardAlgImpl.solveMathExpression(mathExpression, configurationBuilder.isBalanceParenthesesEnabled(), configurationBuilder.getPrecision(), configurationBuilder.isUseRadiansEnabled());
     }
 }