http_output keystore_password is required if keystore is specified

[regulatre]

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For all general issues, please provide the following details for fast resolution:

• Version: 7.6 (Docker image docker.elastic.co/logstash/logstash:7.6.2)
• Operating System: Linux

Also described here: elastic/logstash#11110

If a keystore is specified in the http_output configuration then the keystore_password must also be specified otherwise a nilclass error occurs (described in the URL above). Ideally the code should default to a value of "changeit". However until then, it would be beneficial to add to the documentation a note about the keystore_password value being required is keystore is specified.