Log warning when returning null TableMetadata

If user sets connector up in schemaless mode and forgets
to create the table, the connector tasks will fail with NullPointerException.
Added warning should make it more obvious as to what is the issue.

I am not adding throw here just in case there exists a situation
where table creation could lag behind. Connector should continue
working normally after the table is created.

src/main/java/io/connect/scylladb/ScyllaDbSessionImpl.java

@@ -91,6 +91,7 @@ public TableMetadata.Table tableMetadata(String tableName) {
         result = new TableMetadataImpl.TableImpl(tableMetadata.get());
         this.tableMetadataCache.put(tableName, result);
       } else {
+        log.warn("Could not find metadata for table {} in keyspace {}. Are you sure the table exists?", tableName, keyspaceMetadata.getName().asCql(false));
         result = null;
       }
     }