Regex optional groups crash

[FF-Brown]

When an optional group was not present in the test string, 'group' was null and caused an exception to be thrown. Now we fall back to an empty string instead.

How to test the PR

1. Open the regex debugger
2. Enter the pattern Hello(World)?$ in Box 1 (as suggested in issue Regex - Optional Capture Group #2002 )
3. Enter the string Hello in Box 2
4. No error is thrown, the string is shown to be a match, and the captured group is displayed
5. Additionally, it works with the test string HelloWorld

Checklist

• [ x ] New functionality has tests.
• [ x ] README has been updated (if necessary). (N/A)
• [ x ] New environment variables have been added to .env.example (if necessary). (N/A)

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