Confusing warning 'calling IsSet with unsupported type "invalid" (<nil>)'

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How to reproduce:

Remove these lines from userguide/hugo.yaml:

docsy/userguide/hugo.yaml

Lines 67 to 84 in 706dbf9

	ui:
	sidebar_menu_compact: true
	sidebar_menu_foldable: false
	sidebar_cache_limit: 10
	breadcrumb_disable: false
	sidebar_search_disable: false
	feedback:
	enable: true
	'yes': >-
	Glad to hear it! Please <a
	href="https://github.com/google/docsy/issues/new">tell us how we can
	improve</a>.
	'no': >-
	Sorry to hear that. Please <a
	href="https://github.com/google/docsy/issues/new">tell us how we can
	improve</a>.
	readingtime:
	enable: false

Start preview of the user guide:

hugo server --themesDir ../..

With configuration parameter params.ui not being present in hugo.yaml any more, a warning message is now emitted:

$ hugo server --themesDir ../..
Start building sites …
hugo v0.111.2-4164f8fef9d71f50ef3962897e319ab6219a1dad+extended windows/amd64 BuildDate=2023-03-05T12:32:20Z VendorInfo=gohugoio
WARNING: calling IsSet with unsupported type "invalid" (<nil>) will always return false.

As pointed out in #607 already, such a warning is too noisy and should be avoided.

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Just realized that this warning is printed out in other contexts as well (e.g. when working with taxonomies). The root cause is the wrong use of function isset.