add default of "" for externalDatabase.host; update README to be a little more clear

[jessebot]

Description of the change

Adds a default for externalDatabase.host to be "" so it's clear the type taken, and also reminds people that they need to quote when using a : in a value in yaml. Also adds a bit more of a comment for that value and updates the README to reflect both changes.

Benefits

Just resolves tiny bit of possible confusion around how to specify a database port.

Possible drawbacks

none that I can think of

Applicable issues

• fixes #632
• addresses https://github.com/nextcloud/helm/discussions/618

Checklist

• [x] I have read the CONTRIBUTING.md doc.
• [x] DCO has been signed off on the commit.
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] (optional) Parameters are documented in the README.md