Allow null values in setVar method (#73)

* Added handler to define not found variables
Added support for string variables
Fixed strings and ints comparison error

* Check if variables have scalar types (int, float, string and bool)
Better $onVarNotFound logic

* Better support for null variables

* Better support for null variables

* Better support for null variables

* Allow null values in `setVar` method

src/NXP/MathExecutor.php

@@ -431,7 +431,7 @@ public function getVar(string $variable)
      */
     public function setVar(string $variable, $value) : self
     {
-        if (!is_scalar($value)) {
+        if (!is_scalar($value) && $value !== null) {
             $type = gettype($value);
             throw new MathExecutorException("Variable ({$variable}) type ({$type}) is not scalar");
         }