BUG: Meson-python crashes when specifying an incomplete [tool.meson-python.args] section in pyproj.toml

[peter-urban]

Meson-python allows to specify arguments for meson dist, meson setup, meson compile and meson install using a tool.meson-python.args section in pyproject.toml.

However if this section is created, meson-python will fail if not all four options (dist, setup, compile and install) are explicitly specified in this section.

e.g. the following works:

[tool.meson-python.args] 
dist = []
setup = []
compile = []
install = ["--tags=runtime"]

the following crashes (meson-python: error: Unknown configuration key "tool.meson-python.args.setup"):

[tool.meson-python.args]
dist = []
compile = []
install = ["--tags=runtime"]