fix: doc-explorer search combobox null checks

[LBPeterson]

This PR adds a null check to fix a bug in the doc explorer that happens when the search field loses focus when there is no text in the search input. To reproduce in the Graphiql live demo:

• Open the browser console
• Open the Documentation Explorer (upper left)
• Start searching in the pane
• Delete your search term and click away from the search bar
• See the console log this error: Uncaught TypeError: right-hand side of 'in' should be an object, got null

The cause: The headlessui combobox is nullable by default in ^2, a change in api from ^1 Our existing onChange handler does not expect null values to be passed

The fix: Add an early null check to the onChange handler

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 6a8d630195e6edd2aa65a20d4358ccd2aeac13f7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@graphiql/plugin-doc-explorer	Patch

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[linux-foundation-easycla[bot]]

• :x: - login: @LBPeterson / name: Luke B. Peterson . The commit (8bc509e8b646e3494b05e5310e82f4edcbd2710d, 6a8d630195e6edd2aa65a20d4358ccd2aeac13f7) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[dimaMachina]

closing due missing CLA

this issue will be fixed in the upcoming GraphiQL 5, which will be released tomorrow https://github.com/graphql/graphiql/pull/4030