fix: deploy_filename errors when no deployment.md is found.

[Josverl]

Found while iterating over all boards.
An IndexError: list index out of range is raised when accessing deploy_filename for a board that has no deploy.md

( unix, windows , webassembly)

[andrewleech]

I've run into this issue myself, however it looks like the use of this property still expects it to be a path?

mpbuild/src/mpbuild/build.py

Line 189 in d1c200e

if _board.deploy_filename.is_file():

[Josverl]

I've ran the modified versions a few hundred times across all versions of MicroPython, and found no issues.

But I've been adding on as additional functionality as well, that could have hidden the issue. I'll take a look to make sure.