Allow negative interest rate in PPMT() Financial function (#2164)

src/PhpSpreadsheet/Calculation/Financial/CashFlow/CashFlowValidations.php

@@ -15,9 +15,6 @@ class CashFlowValidations extends FinancialValidations
     public static function validateRate($rate): float
     {
         $rate = self::validateFloat($rate);
-        if ($rate < 0.0) {
-            throw new Exception(Functions::NAN());
-        }
 
         return $rate;
     }

tests/data/Calculation/Financial/PPMT.php

@@ -5,6 +5,10 @@ return [
         -75.623186008367,
         [0.10 / 12, 1, 2 * 12, 2000],
     ],
+    [
+        -91.597714484189,
+        [-0.10 / 12, 1, 2 * 12, 2000],
+    ],
     [
         -27598.053462421,
         [0.08, 10, 10, 200000],