Summary

Per docs (https://docs.mongodb.com/manual/core/read-preference-tags), an empty tag can be used as a "fail-safe" mechanism  to connect to any node regardless of tag:

"To avoid an error if no members match any of the tag specifications, you can add an empty document {{{ }}} as the last element of the tag set to read from any eligible member."

However, when using "readPreferenceTag=" as the last tag set in the connection URI the driver fails to send an empty document in the tag array.  I found a commit (https://github.com/mongodb/mongo-go-driver/commit/bfd961abf162856d6dc7c6d97ec627c18e1ceed0) that seems to simply break instead of treating as an empty document.

How to Reproduce

1. Standup replica set and provide any tag (ex: {dc: dev} )
2. Try to connect with URI containing mismatched tag, but with empty tag as a "fail-safe" (ex: ?readPreferenceTag=dc:prod&readPreferenceTag=)
3. Connection will fail