Update src/RegexMatchValidator.php

Co-authored-by: Eric Lippmann <eric.lippmann@icinga.com>

Author: Al2Klimov

src/RegexMatchValidator.php

@@ -47,7 +47,7 @@ public function isValid($value): bool
         $this->clearMessages();
 
         if (! preg_match($this->pattern, $value)) {
-            if (empty($this->notMatchMessage)) {
+            if ($this->notMatchMessage === null) {
                 $this->addMessage(sprintf(
                     $this->translate("'%s' does not match against pattern '%s'"),
                     $value,