Make the backend more robust concerning unexpected schemas

+++ b/src/SolrConnector/SolrConnectorPluginBase.php
@@ -420,7 +420,7 @@ public function getSchemaVersionString($reset = FALSE) {
   public function getSchemaVersion($reset = FALSE) {
     $parts = explode('-', $this->getSchemaVersionString($reset));
-    return $parts[1];
+    return isset($parts[1]) ? $parts[1] : '';
   }

Exploding any kind of version string coming from a different server might set $parts[1] and therefor mess up things.
I suggest to verify the schema string if it matches our pattern.
If not this function should return something like const FOREIGN_SCHEMA.

It seems that it makes a lot of sense to clearly indicate that we discovered a foreign schema and to deal with it at various places.

the code in question is now part of the connector.

There's a checkbox in the server settings to turn off the schema check.