Missing-requirements error shows an empty directory ("does not exist in ''") for a bare notebook filename #896

🤖 When a notebook is given as a bare filename (relative path, no directory), the missing-requirements error shows an empty directory.

Repro

mkdir p && cd p
echo '{"cells":[],"metadata":{},"nbformat":4,"nbformat_minor":5}' > nb.ipynb
rsconnect write-manifest notebook nb.ipynb      # same for deploy notebook / deploy quarto with a bare filename

Error: The requirements file 'requirements.txt' does not exist in ''.

The directory comes from os.path.dirname("nb.ipynb"), which is ''. With an absolute path, the message shows the correct directory.

Expected

... does not exist in '.' or the absolute directory.

Found during live QA of #892 against Posit Connect. Reproduced on 1.32.0 and current main (0287aca3).