change from present? to ruby-only check

I'm not sure where `present?` (a method defined in Rails) is being pulled in from, but for me, it's not:

```sh
NoMethodError: undefined method `present?' for nil (NoMethodError)

        elsif multi_tenant.present?
                          ^^^^^^^^^
```

I wonder if `elsif multi_tenant` is good enough, or do we need to explicitly check that if it has an `empty?` method, that the `empty?` method returns false (similar to what `present?` does)

Author: mculp

lib/weaviate/schema.rb

@@ -45,7 +45,7 @@ def create(
         req.body["properties"] = properties unless properties.nil?
         if multi_tenant.is_a?(Hash)
           req.body["multiTenancyConfig"] = multi_tenant
-        elsif multi_tenant.present?
+        elsif multi_tenant && !(multi_tenant.respond_to?(:empty?) && multi_tenant.empty?)
           req.body["multiTenancyConfig"] = {enabled: true}
         end
         req.body["invertedIndexConfig"] = inverted_index_config unless inverted_index_config.nil?