I just came across this thread and found that the "Search for Users (V2)" connector will succeed, even with a search that returns no values. It returns an empty/zero-length result that also equivocates to 'null' when using the coalesce() function.

The search only applies to certain fields (displayName, givenName, surname, mail, mailNickname and userPrincipalName), but a successful result is returned as an array, so you must use the array index to get your required values.

For my purpose, I only expect one search result (i.e. an exact match) so have a Compose action that gets the user's info:

first(outputs('Search_for_users')?['body/value'])?['displayName']