Steps to reproduce the problem:

emacs -Q
C-x b "Sample" RET (that is, create an empty buffer)
M-x diff-buffer-with-file RET
RET (confirm the default buffer)

Actual result:

Emacs shows "Wrong type argument: stringp, nil"

Expected result:

Emacs shows a more descriptive error that explains why the command
couldn't be executed.

I've attached a patch that implements the expected result.

Thank you very much.