Add guard-clause to clearDirty method (#16467)

### What does it do?
This makes sure the JS doesn't throw an error in the case that the
items-object is an array, just like in
#16404 for 3.x

### Why is it needed?
When using ExtJS to create components of the xtype: 'radiogroup' to be
used in CMPs inside a form,
this function gets called and tries to iterate over it's items array
with a method that doesn't exist.

This PR isn't changing the way forms or the clearDirty-method are
handled, it just assures no error is thrown when using
radiogroup-components.

### Related issue(s)/PR(s)
Resolves modmore/ClientConfig#202
Resolves modmore/ClientConfig#176
Resolves modmore/ClientConfig#143

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Co-authored-by: Jason Coward <jason@opengeek.com>