deal with nil document types in Multimodel

[catwell]

The simplest solution to deal with the removal of types in ElasticSearch is simply to use single-type indices and not set document_type on models.

However Multimodel did not deal with that case and ended up calling a URL ending in my-index//_search instead of my-index/_search, which caused the types warning:

warning: 299 Elasticsearch-7.9.1-083627f112ba94dffc1232e8b42b73492789ef91 "[types removal] Specifying types in search requests is deprecated."

This PR fixes that issue.

[cla-checker-service]

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[picandocodigo]

Thank you for this contribution @catwell!