Avoid type errors when public key is not retrieved

When dealing with unstable connections, phpseclib may return `false`
when retrieving the public key.  That causes `null` to be passed down to
`base64_decode()` and triggers a TypeError due to strict types.

This exits early, preventing triggering the problem. However, testing is
a bit tricky since we need an unstable connection.

src/PhpseclibV2/SftpConnectionProvider.php

@@ -143,7 +143,12 @@ private function checkFingerprint(SFTP $connection): void
             return;
         }
 
-        $publicKey = $connection->getServerPublicHostKey() ?: 'no-public-key';
+        $publicKey = $connection->getServerPublicHostKey();
+
+        if ($publicKey === false) {
+            throw UnableToEstablishAuthenticityOfHost::becauseTheAuthenticityCantBeEstablished($this->host);
+        }
+
         $fingerprint = $this->getFingerprintFromPublicKey($publicKey);
 
         if (0 !== strcasecmp($this->hostFingerprint, $fingerprint)) {

src/PhpseclibV3/SftpConnectionProvider.php

@@ -150,7 +150,12 @@ private function checkFingerprint(SFTP $connection): void
             return;
         }
 
-        $publicKey = $connection->getServerPublicHostKey() ?: 'no-public-key';
+        $publicKey = $connection->getServerPublicHostKey();
+
+        if ($publicKey === false) {
+            throw UnableToEstablishAuthenticityOfHost::becauseTheAuthenticityCantBeEstablished($this->host);
+        }
+
         $fingerprint = $this->getFingerprintFromPublicKey($publicKey);
 
         if (0 !== strcasecmp($this->hostFingerprint, $fingerprint)) {