Fix downward migrations / allow initialization without both ClientSettings if ConnectionString and DatabaseName have been provided

Mongo.Migration/Migrations/Database/DatabaseMigrationRunner.cs

@@ -97,6 +97,11 @@ private void MigrateDown(IMongoDatabase db, DocumentVersion currentVersion, Docu
                 {
                     break;
                 }
+                else if (migration.Version > currentVersion) // a migration that was never applied
+                {
+                    this._logger.LogInformation("Skipping Migration Down: {0}:{1} because current version is {2} ", migration.GetType().ToString(), migration.Version, currentVersion);
+                    continue;
+                }
 
                 this._logger.LogInformation("Database Migration Down: {0}:{1} ", migration.GetType().ToString(), migration.Version);
 

Mongo.Migration/Migrations/Database/StartUpDatabaseMigrationRunner.cs

@@ -27,7 +27,7 @@ public StartUpDatabaseMigrationRunner(
                 collectionLocator,
                 migrationRunner)
         {
-            if (settings.ConnectionString == null && settings.Database == null || settings.ClientSettings == null)
+            if ((settings.ConnectionString == null || settings.ClientSettings == null) && settings.Database == null)
             {
                 throw new MongoMigrationNoMongoClientException();
             }

Mongo.Migration/Migrations/Document/StartUpDocumentMigrationRunner.cs

@@ -34,7 +34,7 @@ public StartUpDocumentMigrationRunner(
                 documentVersionService,
                 migrationRunner)
         {
-            if (settings.ConnectionString == null && settings.Database == null || settings.ClientSettings == null)
+            if ((settings.ConnectionString == null || settings.ClientSettings == null) && settings.Database == null)
             {
                 throw new MongoMigrationNoMongoClientException();
             }