custom_target: install:true without install_dir crashes

[mon]

Describe the bug
When defining a custom_target with install:true, meson build will crash. If install_dir is provided, build works as expected.

Traceback:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/mesonmain.py", line 138, in run
    return options.run_func(options)
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/msetup.py", line 294, in run
    app.generate()
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/msetup.py", line 185, in generate
    self._generate(env)
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/msetup.py", line 247, in _generate
    intr.backend.generate()
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/backend/ninjabackend.py", line 557, in generate
    self.generate_install()
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/backend/ninjabackend.py", line 1120, in generate_install
    self.create_install_data_files()
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/backend/backends.py", line 1495, in create_install_data_files
    pickle.dump(self.create_install_data(), ofile)
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/backend/backends.py", line 1483, in create_install_data
    self.generate_target_install(d)
  File "/usr/local/lib/python3.8/dist-packages/mesonbuild/backend/backends.py", line 1627, in generate_target_install
    dir_name = os.path.join('{prefix}', outdir)
  File "/usr/lib/python3.8/posixpath.py", line 90, in join
    genericpath._check_arg_types('join', a, *p)
  File "/usr/lib/python3.8/genericpath.py", line 152, in _check_arg_types
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

To Reproduce

project('test')

custom_target(
    output: 'bug.txt',
    install: true,
    capture: true,
    command: ['echo', 'foobar']
)

Expected behavior
Either an error of some sort ("install:true requires install_dir to be set") or default fallback (install_dir: '.' ??)

system parameters

• Native build
• Ubuntu 20.04
• Python 3.8.10
• Using the commit at time of bug report: d23ae8b
• Ninja 1.10.0

[eli-schwartz]

This happens because some code, but not all code, is migrated to use typed_kwargs. And the CustomTarget implementation just emits the relevant error if install_dir is not in the kwargs, but it might be passed and not truthy.

Seems like a simple solution.