PHP 8.5 deprecations in the multisite clone form [#3626541]

Problem/Motivation

On PHP 8.5, the "Clone for multisite" index form (/admin/config/search/search-api/index/{index}/solr_multisite_clone) triggers the following deprecations:

Deprecated function: Using null as an array offset is deprecated, use an empty string instead in Drupal\search_api_solr\SolrFieldManager->getFieldDefinitions() (line 80 of modules/contrib/search_api_solr/src/SolrFieldManager.php).
On submitting it: Method ReflectionMethod::setAccessible() is deprecated since 8.5 in IndexSolrMultisiteCloneForm::submitForm(). Reflection is still needed because Index::writeChangesToSettings() is protected (see #3020065), but setAccessible() has had no effect since PHP 8.1.

Steps to reproduce

On PHP 8.5, open an index's "Clone for multisite" form, then submit it.

Issue fork search_api_solr-3626541

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