fix: the error 'an unexpected error occured "" [] false' for an empty formula

Author: optimistex

expression.php

@@ -171,6 +171,9 @@ public function e($expr)
      */
     public function evaluate($expr)
     {
+        if (empty($expr)) {
+            return false;
+        }
         $this->_functions = array_merge($this->_functions, $this->functions);
         $this->last_error = null;
         $expr = preg_replace("/\r|\n/", '', trim($expr));

tests/Expression.php

@@ -27,6 +27,7 @@ public function testEmptyFormula()
         $expr = new Expression();
         $expr->suppress_errors = true;
         $this->assertEquals($expr->evaluate(''), false);
+        $this->assertEmpty($expr->last_error);
     }
 
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