Find and replace next/previous buttons are active even when no text is in the find textbox

[JasonWeill]

Originally reported by @tgeorgeux in #6650, sub-issue 1.

The "next match" and "previous match" buttons in the search and replace UI control are always active, even when no text is in the Find textbox:

These buttons are active, and they can be pressed, when nothing is entered in the search text box. They also cause JavaScript errors like the following when pressed, although they do not scroll the page, as reported in #6650-1. (The error below was thrown in code on the JupyterLab tip of main, in Lab 4.1.)

Uncaught (in promise) TypeError: searchEngine is undefined
    _stepNext packages_notebook_lib_index_js.ca88e975802794059d81.js:6115
    highlightNext searchprovider.ts:327
    highlightNext searchmodel.ts:273
    onHighlightNext searchview.tsx:917
    onHighlightNext searchview.tsx:723
    onClick searchview.tsx:322
    React 23
    renderDOM vdom.ts:82
    renderDOM vdom.ts:79
    onUpdateRequest vdom.ts:51
    processMessage index.es6.js:1206
    invokeHandler index.es6.js:412
    sendMessage index.es6.js:166
    onAfterAttach vdom.ts:59
    processMessage index.es6.js:1240
    invokeHandler index.es6.js:412
    sendMessage index.es6.js:166
    attach index.es6.js:1663
    getSearchWidget index.ts:308
    execute index.ts:329
    execute index.es6.js:364
    _executeKeyBinding index.es6.js:526
    processKeydownEvent index.es6.js:462
    evtKeydown index.es6.js:2111
    handleEvent index.es6.js:2067
    addEventListeners index.es6.js:2098
    node_modules jlab_core.3a6c2825271f06388f85.js:4616
    promise callback*start index.es6.js:2021
    main index.out.js:701
    bootstrap bootstrap.js:96
    js bootstrap.js:97
    Webpack 5
3 packages_notebook_lib_index_js.ca88e975802794059d81.js:6115:1