fix: Return this if already loaded

[benjackwhite]

Changes

Related to #1090

We don't return posthog if the SDK is already initialised or if the token is missing. This appears to be a regression

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

[github-actions]

Size Change: +64 B (0%)

Total Size: 941 kB

Filename	Size	Change
dist/array.full.js	224 kB	+16 B (0%)
dist/array.js	124 kB	+16 B (0%)
dist/es.js	124 kB	+16 B (0%)
dist/module.js	124 kB	+16 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12.1 kB
dist/recorder-v2.js	106 kB
dist/recorder.js	106 kB
dist/surveys-module-previews.js	62 kB
dist/surveys.js	58.3 kB

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