Fix schema validation: The URL property of achievement is optional

[McPringle]

The URL property of achievement is optional, but the schema validation requires this property:

[[12](https://github.com/hackergarten/hackergarten.github.io/actions/runs/3393744962/jobs/5641443017#step:4:13):[22](https://github.com/hackergarten/hackergarten.github.io/actions/runs/3393744962/jobs/5641443017#step:4:23):44] Using gulpfile ~/work/hackergarten.github.io/hackergarten.github.io/gulpfile.js
[12:22:44] Starting 'default'...
[12:22:44] Starting 'generate-xml'...
[12:22:44] Starting 'validate-events'...
[12:22:44] ✖ events.json has 1 validation error(s)
[12:22:44]    › Error: Missing required property: url
[12:22:44]      at /5/achievements/3 against schema /items/properties/achievements/items/required/1
[12:22:44] Schema validation failed for 1 file(s) / 1 file(s)
[12:22:44] 'validate-events' errored after 106 ms
[12:22:44] Error in plugin 'gulp-json-schema'
Message:
    There was schema validation error
Details:
    domainEmitter: [object Object]
    domainThrown: false
[12:22:44] 'generate-xml' errored after 108 ms
[12:22:44] 'default' errored after 109 ms

The schema validation should be modified in a way that the URL property of achievement is optional. The URL property a link is still required.

[McPringle]

Fixed with PR #416