Fix wrong v2 hash string displayed in WebUI

Previously `0000...` was erroneously displayed when v2 hash is absent,
now it correctly shows the `N/A`.

PR #16846.

src/base/digest32.h

@@ -122,7 +122,7 @@ public:
 
     QString hashString() const
     {
-        if (m_hashString.isEmpty())
+        if (m_hashString.isEmpty() && isValid())
         {
             const QByteArray raw = QByteArray::fromRawData(m_nativeDigest.data(), length());
             m_hashString = QString::fromLatin1(raw.toHex());