After #4077, basic_auth in HTTP configs now requires setting a username, otherwise you get an error:

[...] err="Error loading config couldn't load configuration (--config.file=prometheus.yml): parsing YAML file prometheus.yml: basic_auth requires a username"

Using basic auth without a username has been standard practice, for example in the suggested remote_write configs for Cortex. So this is a breaking change and needs to be fixed.