fix null values for old_values and new_values (#959)

Author: tminich

src/Audit.php

@@ -102,11 +102,11 @@ public function resolveData(): array
         $this->metadata = array_keys($this->data);
 
         // Modified Auditable attributes
-        foreach ($this->new_values as $key => $value) {
+        foreach ($this->new_values ?? [] as $key => $value) {
             $this->data['new_' . $key] = $value;
         }
 
-        foreach ($this->old_values as $key => $value) {
+        foreach ($this->old_values ?? [] as $key => $value) {
             $this->data['old_' . $key] = $value;
         }