fix(theme): unresponsive back button with empty input in search box

[quadratz]

Steps to Reproduce:

1. Navigate to the page with the local search functionality (not Algolia).
2. Make sure the screen size is small or use a mobile phone.
3. Click on the search box to open it.
4. Without entering any text, click the back button.
5. Observe that the search box does not close.