fix custom_target with install: true and no install_dir, crashing

It's supposed to emit an error message, but instead it did a traceback.
It used to be, if no install_dir was specified then it was simply not in
kwargs, but due to typed_kwargs it will now be there, but not have
viable contents, so the dict membership check got skipped.

Fixes #9522

mesonbuild/build.py

@@ -2457,7 +2457,7 @@ def process_kwargs(self, kwargs, backend):
             if not isinstance(self.install, bool):
                 raise InvalidArguments('"install" must be boolean.')
             if self.install:
-                if 'install_dir' not in kwargs:
+                if not kwargs.get('install_dir', False):
                     raise InvalidArguments('"install_dir" must be specified '
                                            'when installing a target')