All configuration keys must be set if any are set

[ngoldbaum]

Currently, if one has a pyproject.toml that looks like:

[tool.meson-python.args]
setup = ["-Ddebug=true", "-Doptimization=0"]

Then at build time meson-python will raise an error:

meson-python: error: Unknown configuration key "tool.meson-python.args.dist"

A workaround is to set empty values for all supported configuration options:

[tool.meson-python.args]
dist = []
setup = ["-Ddebug=true", "-Doptimization=0"]
compile = []
install = []

It looks like the existing tests only use a pyproject.toml that has all four keys set.

[dnicolodi]

Duplicate of #293. Fix waiting in #304.