On the edit media screen, ensure that the 'replace' button is disabled until a media item is selected. Fix an issue where clicking the button with no item selected threw a JavaScript error and failed to work.
Props timmydcrawford, subrataemfluence, psiico.
Fixes#40589.
git-svn-id: https://develop.svn.wordpress.org/trunk@40902 602fd350-edb4-49c9-b593-d223f7449a82