Interview solutions to reverse a string and reverse a sentence
The reverse of an empty string should be an empty string, not null.

In reverseSentence(), the reversed.trim() call inefficiently copies the entire string, less one final space.  You would have been better off shortening the StringBuilder instead of trimming the string.

All of your uses of final are excessive, I think.  The final keyword merely prevents reassignment, and doesn't make anything more immutable than it already is.  For short functions like this, writing final just adds noise.