[5.0] Fix events with non required arguments: AfterPurgeEvent::setSubject(): Argument 1 must be of type string, null given

[Fedik]

Pull Request for Issue # .

Summary of Changes

reshapeArguments should not be called when there no required arguments, and arguments is empty.

Fix error AfterPurgeEvent::setSubject(): Argument #1 ($value) must be of type string, null given
When trying to clean all cache, and similar cases

Testing Instructions

Go to Maintenance: clear cache.
And click "Delete all"

Actual result BEFORE applying this Pull Request

And error

Expected result AFTER applying this Pull Request

No error

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org:
• No documentation changes for manual.joomla.org needed

[Quy]

I have tested this item ✅ successfully on 5372b6e

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